Incoloy 800H Creep Failure in Furnace Tubes

By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012

Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360

Quick Answer: Why do Incoloy 800H tubes fail in high-temperature service?

They fail by creep, not by corrosion and not by a single overload. At 800 °C and above, a tube deforms continuously under its own pressure and weight, runs out of rupture life, and eventually bulges and splits. The controlling variables are the metal temperature, the stress and the grain size of the tube. Its progress is measurable long before rupture.

Key Takeaways

• The life-limiting mechanism in 800H is creep rupture, and it is a design calculation rather than an operating surprise. Above about 650 °C the allowable stress is set by stress-rupture data, and the design life is the time to rupture at the metal temperature and stress.
• Metal temperature dominates the calculation. Rupture life falls steeply with temperature, so a tube running 20 °C hotter than the design figure loses life far faster than a tube carrying 20 % more stress.
• Grain size is a specification requirement, not a mill preference. The coarse grain size required by the 800H specification is what produces the creep strength, and a tube supplied with a fine grain structure will not reach its design life.
• Bulge measurement is the practical monitoring tool. Progressive diametral strain accumulates before rupture, and periodic measurement finds the tubes that are running out of life while replacement can still be planned.
• Carburisation and sigma phase change the diagnosis. A tube that has carburised or precipitated sigma phase loses ductility and fails earlier than the rupture calculation predicts, and metallography is what separates the two cases.
• Replacement specification decides whether the problem returns. Tube chemistry, grain size, wall thickness and the fabrication method all have to be specified deliberately for elevated-temperature service.

Why These Tubes Fail by Creep Rather Than by Corrosion

A furnace tube in a reformer, a steam methane reformer, an ethylene cracking furnace or a direct reduction plant is designed for a metal temperature between roughly 700 °C and 1000 °C and for a wall thickness that is decided by pressure and by the high-temperature strength of the alloy. At those temperatures, the material does not simply resist load the way it does at ambient temperature: it deforms slowly and continuously, and the accumulated deformation ends in rupture after a time that depends on the temperature, the stress and the microstructure. That behaviour, creep, is the mechanism that limits the life of the component, and every other failure mode — oxidation, carburisation, thermal fatigue, cracking at welds — modifies that life rather than replacing it.

The consequence for the engineering is that a high-temperature tube is a consumable with a calculable life rather than a permanent component. Its design is an exchange of wall thickness for temperature: a thicker wall reduces the stress and extends the life, a higher metal temperature reduces the life and demands a thicker wall or a stronger alloy, and the design decision can only be made if the metal temperature is known. This is why the first question in any assessment of a failed tube is what temperature the tube was actually at, rather than what the process design intended, because the second figure is often several tens of degrees below the first in service.

It also explains why corrosion-oriented thinking misleads buyers of these tubes. A purchaser who specifies 800H on the basis of its chemistry and its corrosion resistance has specified the right alloy family for the wrong reason and may still receive a tube that fails early, because the properties that govern the life of a furnace tube are the high-temperature creep properties, and those depend on the grain size and the solution treatment rather than on the chemistry alone. A tube that meets the composition requirements of the specification and has been processed to a finer grain size than the specification contemplates will pass a certificate check and disappoint in service.

A Composite Case: The Bulged Reform Tube

The case described here is a composite, assembled from the failure pattern we are asked about most often; it does not describe a single client's equipment. A reformer operated with 800H tubes in a fired heater, with a design metal temperature of about 900 °C and a design life of 100,000 hours. At about 60,000 hours, routine inspection found that a group of tubes in one pass had increased in diameter by roughly 3 % of the original outside diameter, while the rest of the furnace was still within 1 %. The affected tubes showed no external cracking, the welds were sound, and the process data for the period showed no excursion that the operators considered abnormal.

The sequence that followed is the standard one and it illustrates the diagnostic logic. Ultrasonic thickness measurement showed wall thinning that was consistent with normal oxidation rather than with localised loss, so corrosion was not the cause. Portable hardness and metallographic examination of an extracted sample showed grain growth compared with the original tube, together with sigma phase in the grain boundaries and carburisation at the inner surface. The temperature profile reconstructed from the heater performance suggested that the affected pass had been running 20 to 40 °C hotter than the average for a substantial fraction of its operating life, and the combination of a higher metal temperature, the loss of ductility from sigma phase and the increased stress from the thinner wall explained the deformation.

Three conclusions followed. The first was that the remaining life of the furnace was not uniform, so a single replacement date based on the average condition would have been wrong in both directions, replacing good tubes and leaving bad ones. The second was that the temperature maldistribution was a heater and flow issue rather than a material issue, and that replacing the tubes without addressing it would reproduce the failure. The third was that the replacement tubes had to be specified with the grain size and the solution treatment requirements stated, because the original tubes had met the chemistry requirements of the specification and had still performed unevenly. The case is worth keeping in mind because it demonstrates the general rule: in high-temperature service, the failure is nearly always the result of accumulated conditions rather than a single event, and the material is one variable among several.

Metallurgical Mechanisms: Cavitation, Grain Growth and Phase Precipitation

Incoloy 800H fails through a small number of mechanisms that occur in characteristic temperature ranges and leave characteristic evidence. Being able to recognise them is what allows an inspection finding to be turned into a diagnosis rather than a guess.

Mechanism	Temperature range where it matters	Effect on the tube	Evidence it leaves
Creep deformation and cavitation	above about 650 °C, increasingly severe above 800 °C	progressive diametral strain; cavitation at grain boundaries before rupture	bulge measurements; grain boundary cavities in metallography
Grain growth in service	above about 900 °C, depending on prior condition	loss of the specified grain structure; changed creep and rupture behaviour	grain size measurement by ASTM E112 on a service sample
Sigma phase precipitation	roughly 600 to 900 °C with long exposure	loss of ductility and toughness; reduced creep ductility	metallographic examination; hardness change
Carburisation	carburising process gases, typically 800–1000 °C	carbon pickup, reduced ductility, changed creep behaviour	carbon analysis by depth; metallographic examination of the inner surface
Internal oxidation and scale spalling	above about 900 °C in oxidising atmospheres	wall thinning, local hot spots, loss of metal	ultrasonic thickness measurement; scale examination
Thermal fatigue	cyclic furnace operation with restrained tubes	cracking, usually at welds and supports	liquid penetrant or dye penetrant examination
Stress relaxation cracking in welds	welded joints in the 600–750 °C range	cracking in the heat-affected zone	metallography and examination of welds
Erosion at supports and guides	local gas velocity and contact points	localised wall loss	thickness mapping around supports

Table note: The temperature ranges shown are indicative of where each mechanism becomes significant and they overlap, which is the point: a tube operating at 900 °C is exposed to creep, to grain growth and to carburisation simultaneously, and each of them reduces the life that the others allow. The identification of the mechanism is performed by metallographic examination to ASTM E3 with grain size determination to ASTM E112, by carbon analysis through the wall thickness, and by hardness testing, and the results should be interpreted together with the measured deformation rather than separately. Our Incoloy product range covers the tube and bar forms used for these applications.

Sigma phase deserves a specific comment because it is the mechanism most often overlooked in an assessment. In the iron-nickel-chromium system, exposure in the intermediate temperature range allows a chromium-rich intermetallic phase to precipitate, and its effect is to reduce ductility and creep ductility rather than to reduce strength. A tube that has precipitated sigma phase may still be within its calculated rupture life and yet fail in a brittle manner during a shutdown or a thermal transient, particularly if it is restrained. This is one reason why a rupture calculation alone is not a complete assessment of a furnace, and why metallographic examination of a sample tube is worth the cost of extracting it.

Design Basis: Rupture Strength and How It Is Established

The design of a high-temperature tube is based on stress-rupture data, and the data that matter are the results of tests run to rupture over thousands of hours at the service temperature. Because such tests cannot be run for the full life of a furnace, the design values are produced by extrapolation, and the extrapolation is the reason the design has uncertainty attached to it.

Temperature band	Governing design consideration	Basis of the allowable stress	Verification
below about 540 °C	yield and tensile strength	room and moderate temperature mechanical properties	ASTM E8/E8M to ASTM B407
about 540–650 °C	onset of time-dependent behaviour	creep and rupture data begin to govern for thick sections	ASTM E139 creep testing
about 650–900 °C	stress-rupture life	rupture data at the service temperature, extrapolated to the design life	ASTM E139, code tabulated values
about 900–1000 °C	rupture life and oxidation	rupture data plus the environmental limit of the alloy	ASTM E139 plus service experience
above about 1000 °C	oxidation and melting limits	the alloy is outside its practical design range	selection of a different material

Table note: The bands indicate where each design consideration dominates and are intended to show why the allowable stress for 800H at high temperature is derived from rupture testing rather than from tensile testing; the governing values are the allowable stresses tabulated in the applicable code for the specification and temperature, and they must be used in place of any figure quoted in an article. As an indication of the magnitude involved, published test data for alloy 800H show that the material will sustain approximately 24 MPa for 10,000 hours at 870 °C, which illustrates the extent to which rupture strength falls with temperature compared with the room-temperature yield strength. Creep and creep-rupture testing is conducted to ASTM E139, and the material requirements for the tube form are those of ASTM B407 and the corresponding ASME designation.

The practical consequence of the steep temperature dependence is that temperature measurement and control are as important as material selection, and that a small systematic temperature error has a large effect on life. A tube operating 30 °C above its design metal temperature can consume its rupture life several times faster than the design assumed, and the excess is usually not visible in the process data because the thermocouple that measures the average does not measure the hottest tube. This is why furnace assessments rely on the condition of the metal — bulge measurement, metallography, thickness — rather than only on the recorded process variables, and why the reconstruction of the temperature history is the most valuable part of a failure investigation.

Grain size is the material variable that most affects the rupture life within a given specification. The 800H and 800HT designations exist in part because a controlled coarse grain size improves creep and rupture performance, and the specification therefore requires the material to be solution treated and to have a grain size coarser than a stated limit, which places the requirement on the heat treatment rather than on the analysis. Where tubes are supplied with a finer grain structure than the specification contemplates, the chemistry certificate will show full compliance and the rupture behaviour will be worse than the design assumed. That is why the grain size result belongs on the mill certificate and why it should be checked on receipt, a point our materials verification articles develop for other product forms.

Inspection: What Detects an Ageing Tube

Creep damage develops slowly and leaves measurable evidence at every stage, which is why furnace tubes are monitored rather than simply replaced on a schedule. The measurements that matter are those that detect deformation, wall loss, and the microstructural changes that reduce the remaining life, and each of them addresses a different part of the assessment.

Inspection	What it detects	Method	Limitation
Diameter and bulge measurement	accumulated creep strain	measurement at fixed reference points, recorded by position	requires a baseline and consistent technique
Ultrasonic thickness measurement	wall loss by oxidation and erosion	ultrasonic testing, grid over the tube length	does not detect creep damage before deformation
Hardness testing	condition change, sigma precipitation	portable hardness testing	indicative only; not a life calculation
Replica metallography	cavitation, grain growth, sigma phase	surface replication and examination	shows only the surface, needs experienced interpretation
Destructive metallography	full microstructural assessment	sample removal, ASTM E3 and E112	requires a tube to be taken out of service
Carbon analysis through the wall	carburisation	laboratory analysis of a cross-section	requires a sample
Weld examination	cracking in welds and heat-affected zones	penetrant testing, metallography	welds are only part of the tube length
Temperature survey	the cause of the accelerated damage	thermocouples, infrared survey, process data	fired heaters are difficult to survey reliably
Stress-rupture assessment	remaining life estimate	Larson-Miller or equivalent using measured temperature and stress	the extrapolation carries uncertainty
Oxide scale examination	oxidation rate and spalling behaviour	visual and metallographic examination	indicates environment rather than remaining life

Table note: The methods listed are complementary rather than alternative, and the pairing that gives the best value in practice is periodic bulge measurement for the population of tubes and destructive metallography on a small number of extracted samples to establish the condition of the population. Where the assessment is used to plan a replacement, the measurement records should be traceable to identified tube positions over time, because a single set of diameter readings without a baseline establishes deformation but not the rate at which it is accumulating. Our heat exchanger tube selection article covers the related question of tube specification and quality for lower-temperature duties.

The reason creep damage can be detected at all is that its progression is ordered. The sequence runs from grain boundary cavitation, through the linking of cavities into microcracks, to macro-cracking and finally to rupture, and each stage is accompanied by measurable deformation. That is why a programme of diameter measurement at fixed positions is worth more than an occasional detailed examination: it detects the rate of change, which is what allows the remaining life to be estimated, and the estimate can then be confirmed by metallography on a sample tube taken from the most affected position.

Where the assessment finds that the deformation is general rather than local, the usual explanation is a plain excess of temperature or stress over the whole pass, and the response is a design review. Where it finds that the deformation is local — in one or two tubes of a pass, or at particular elevations — the explanations to consider are flow maldistribution, internal deposits or catalysts, flame impingement, or a local reduction in wall thickness. The distinction changes the remedy, and it can be made from the measurement data before any metallography is performed.

Root Causes in Practice

A creep failure in a furnace tube is almost always attributable to one of a small number of causes, and identifying which one is present determines whether a replacement will last or will repeat the failure. The table below maps the observed condition to the likely root cause.

Observation	Likely root cause	Corrective action
Uniform bulging across a whole pass	metal temperature above design for a sustained period	review firing and flow distribution before replacing tubes
Deformation in individual tubes only	flow maldistribution, restriction or local flame impingement	investigate the specific tube and the burner or distribution arrangement
Cracking concentrated at welds	welding procedure or stress relaxation cracking	qualify a suitable procedure and review the heat treatment of welds
Very low ductility with a brittle fracture appearance	sigma phase precipitation	review the temperature history and the shutdown procedure
Carbon pickup through the wall	carburising process atmosphere	review the alloy selection and the process conditions
Wall thinning with scale spalling	oxidation at excessive temperature	review temperature and surface condition
Failure shortly after commissioning	grain size, solution treatment or wall thickness not as specified	verify the incoming material against the specification
Repeated failures at equivalent locations	design stress too high for the actual temperature	re-rate the design or select a stronger alloy
Early failure after a rebuild	substitute tubes from a different source or specification	enforce the specification and verify on receipt
Failure during a start-up or shutdown transient	thermal fatigue combined with low ductility	review the operating procedure and consider a more tolerant alloy

Table note: The mapping follows the diagnostic logic used in furnace assessments and is intended as a starting point for an investigation rather than a substitute for one, because several of the causes can be present at the same time and the condition observed at failure is the result of their combined effect over the life. The corrective action column is deliberately placed after the root cause rather than alongside the observation, because replacing tubes without establishing why the original ones failed is the most common way of paying for the same failure twice. Our purchase specification guide covers the clauses that prevent the material-related causes recurring on the replacement order.

Two of these entries deserve emphasis. The first is the material cause, because it is the one the buyer can control directly: a tube supplied with a grain size finer than the specification requires, or with a wall thickness at the low end of the tolerance in a design that had no margin, will fail early regardless of how well the furnace is operated. Verification of grain size and wall thickness on receipt is cheap, and it addresses a class of failure that no amount of process adjustment can prevent. The second is the operational cause, because it is the one that is most often left unaddressed: a furnace whose temperature distribution has drifted will consume the rupture life of one pass much faster than another, and a replacement programme that ignores the distribution simply produces another set of unevenly aged tubes.

Prevention: Specification, Fabrication and Operation Controls

Preventing a recurrence means controlling the three things that determine the life of the tube: the material as delivered, the fabrication of the assembly, and the conditions under which it operates. Each of them can be written into a document that is checkable.

Control point	Requirement	What it prevents
Grade and specification	800H to ASTM B407, or 800HT where higher creep strength is required	wrong alloy or wrong specification level
Grain size	coarser than the minimum stated in the specification, verified to ASTM E112	tubes with insufficient creep strength
Solution treatment	as required by the specification, on the certificate	material not processed for creep service
Chemistry	within the specification, including aluminium and titanium where applicable	heat-to-heat variation in creep performance
Wall thickness	minimum wall stated, with the tolerance and the measurement basis agreed	tubes at the low end of the tolerance
Tube straightness and ovality	within limits, to allow correct fitting	eccentric loading at the tube supports
Welding procedure	qualified for the grade and the service, with heat treatment defined	weld failures and stress relaxation cracking
Filler metal	matching specification and certificate	incorrect weld chemistry
Post-weld heat treatment	defined, applied and recorded	residual stress and cracking in service
Documentation	EN 10204 3.1 or 3.2 with grain size and heat treatment records	unverifiable material
Installation	alignment, support condition and clearances as designed	local stress concentrations
Operating limits	maximum tube metal temperature and its monitoring	accelerated life consumption
Inspection programme	baseline measurement, then periodic measurement at fixed positions	undetected progression to rupture

Table note: The controls listed are those that a purchaser, a fabricator and an operator can each influence, and the division between them is worth keeping clear because responsibility for a failure is usually shared across all three. The material controls are checkable at goods-in, the fabrication controls are checkable in the shop, and the operating controls require a measurement programme that is only effective if it has a baseline. Our Incoloy range is supplied with grain size and heat treatment data on the certificate, and our related furnace service comparison covers the alternative grades for the same duty.

The single control with the largest effect per unit of cost is the grain size verification. It is a measurement that takes one test piece and one metallographic examination per heat, it is required by the specification in any case, and it distinguishes tubes that will reach their design life from tubes that will not. Because the requirement sits with the heat treatment rather than with the chemistry, it is also the requirement most likely to be missed by a supplier who is working to a chemistry certificate, which is a reason to state it explicitly in the enquiry and to require the result on the documentation.

The second most valuable control is the baseline measurement programme, and its value is independent of the material. A furnace for which no baseline exists can be assessed only by the absolute diameter of its tubes, which is a weak indicator because the original diameter may not be known precisely and the tolerance may be a fraction of the deformation that matters. A furnace with a documented baseline and periodic measurements at fixed positions can be assessed by the rate of deformation, which allows the remaining life to be estimated and the replacement to be planned rather than discovered.

Replacement and Upgrade Selection

Selecting the replacement is a question of how much creep strength is needed, how much oxidation resistance is needed, and how much the answer costs. The table below compares the grades that are normally considered for furnace tube and high-temperature internals duty.

Grade	Principal advantage	Practical temperature range	Where it is preferred	Form and availability
Incoloy 800H	balanced creep strength, oxidation and cost	to about 900 °C, higher with acceptance	general furnace and reformer tube duty	tube, plate, bar, forgings
Incoloy 800HT	higher creep strength than 800H	to about 950 °C	severe reformer service where 800H life is marginal	tube, bar
Inconel 601	oxidation resistance and strength	to about 1000 °C	high-temperature internals in oxidising atmospheres	tube, plate, bar
Inconel 617	high creep strength and oxidation resistance	to about 1000 °C	advanced high-temperature components	plate, bar, tube
Inconel 625	creep strength plus corrosion resistance	to about 700 °C continuous	high-temperature service with a corrosive component	tube, plate, bar
Alloy 310S / 253MA	lower cost heat-resistant stainless	to about 900 °C with lower strength	low-stress internals and supports	tube, plate, bar
Centrifugally cast HK40 / HP-modified	highest creep strength per unit cost at temperature	900–1100 °C	pyrolysis and reformer tubes in severe duty	cast tube
Alloy 20 / C-276	corrosion resistance rather than creep strength	lower temperature with aggressive media	chemical process heater tubes	tube, plate, bar

Table note: The temperature ranges are indicative of practical service limits for each family and are not design allowables; the design value for any of these grades is the allowable stress tabulated in the applicable code for the specification, temperature and product form. The comparison between a wrought tube and a centrifugally cast tube is a separate decision, because the cast grades offer substantially higher creep strength at the highest temperatures while the wrought grades offer better ductility, weldability and availability. Where the duty alternates between a corrosive and a high-temperature condition, the selection should be made on the mechanism that limits life rather than on the one that is easier to measure.

Product	Grade	Reference range, 2026, EXW Shanghai	Note
Seamless tube, 50–150 mm OD	Incoloy 800H	USD 26–48/kg	wall thickness and quantity drive the band
Seamless tube, 50–150 mm OD	Incoloy 800HT	USD 30–55/kg	tighter chemistry and controls
Seamless tube, 50–150 mm OD	Inconel 601	USD 45–75/kg	higher nickel and chromium content
Seamless tube, 50–150 mm OD	Inconel 617	USD 70–120/kg	low availability, made to order
Seamless tube, 50–150 mm OD	Alloy 310S	USD 8–16/kg	for comparison on lower-stress duty
Plate and bar	Incoloy 800H	USD 22–40/kg	for internals, supports and flanges
Centrifugal cast tube	HK40 / HP-modified	quotation by size and length	castings priced individually
Fabrication	tube welding, bending, heat treatment	quotation by joint and operation	procedure qualification included where required

Table note: Reference range only, 2026, EXW Shanghai, USD/kg, subject to the LME nickel price and to the chromium and molybdenum markets; these figures are indicative and are not a quotation, and actual prices depend on specification, tolerances, length, quantity, testing and documentation. The comparison that matters in a furnace replacement is not the price per kilogram but the cost per unit of tube per year of life, which requires the rupture life at the actual operating temperature to be calculated for each candidate grade. Our price benchmark article and our Inconel range provide the wider context, and our Chinese supply overview covers the commercial and documentation aspects of sourcing these grades.

Standard Index

Standard	Title / scope	Covers	Form
ASTM B407	Nickel-iron-chromium alloy (UNS N08800, N08810, N08811) seamless tube and pipe	composition + mechanical	tube, pipe
ASTM B408	Nickel-iron-chromium alloy rod and bar	composition + mechanical	bar, rod
ASTM B409	Nickel-iron-chromium alloy plate, sheet and strip	composition + mechanical	plate, sheet
ASTM B515	Welded UNS N08120, N08800, N08810 and N08811 tube	composition + mechanical	welded tube
ASME SB407 / SB408 / SB409	ASME adoption of the above for pressure equipment	composition + mechanical	tube, bar, plate
ASME BPVC Section II Part D	Properties of materials, including allowable stresses at temperature	design values	all forms
ASTM E139	Conducting creep, creep-rupture and stress-rupture tests	test method	—
ASTM E8 / E8M	Tension testing of metallic materials	test method	—
ASTM E21	Elevated-temperature tension testing	test method	—
ASTM E112	Determining average grain size	test method	—
ASTM E3	Preparation of metallographic specimens	test method	—
ASTM E1476 / E572	Metals identification and analysis of nickel alloys	test method	—
ISO 15614-1 / ASME Section IX	Welding procedure qualification	qualification	all forms
EN 10204	Metallic products — types of inspection documents	inspection documents	all forms

Table note: Standards are listed by number and scope; where an edition year is not quoted the latest edition applies, and the controlling edition is the one named on the purchase order. The distinction that matters for a high-temperature tube is between the product specification, which defines the composition, the mechanical properties and the grain size that the tube must have on delivery, and the design code, which supplies the allowable stress at temperature; a certificate that demonstrates compliance with the first says nothing about the second, and the design calculation must be made separately.

FAQ

Q1: What temperature does Incoloy 800H creep become a design concern?

Creep becomes a design consideration at lower temperatures than most buyers expect: time-dependent behaviour starts to matter in the region of 540 to 650 °C, depending on stress and section thickness, and above about 650 °C the allowable stress for the material is derived from stress-rupture data rather than from tensile data. Incoloy 800H is normally selected for service between roughly 650 °C and 900 °C, with the upper end of that range requiring a design based on rupture life and oxidation allowance rather than on strength alone. The practical question is not the temperature at which creep begins but the temperature at which the rupture life at the design stress becomes shorter than the design life of the equipment, because that is the point at which the grade, the wall thickness or the operating conditions have to be reconsidered. Above about 950 °C the selection usually moves to a higher-strength alloy or to a cast tube grade.

Q2: How is the remaining life of a furnace tube estimated?

By combining the measured condition of the metal with a rupture calculation. The condition is established from diameter measurements at fixed positions over time, from ultrasonic thickness measurements, and from metallographic examination of a sample tube that identifies cavitation, grain growth and phase precipitation. The calculation uses the measured or reconstructed metal temperature and the calculated stress from pressure and weight, with a relationship such as the Larson-Miller parameter to relate the service condition to rupture data. The result is an estimate with a real uncertainty attached to it, and it should be treated as a planning tool rather than a prediction. Where the estimate and the physical evidence disagree, the physical evidence governs. Our materials and inspection articles cover the verification methods involved.

Q3: Why does grain size matter so much in 800H?

Because the creep strength of the alloy depends on it. Creep deformation in these materials occurs largely by grain boundary sliding and by the movement of dislocations, and a coarser grain structure reduces the total grain boundary area available for sliding, which improves the creep and rupture performance. The 800H and 800HT designations exist precisely because a controlled coarse grain structure, produced by solution treatment at high temperature with rapid cooling, gives better elevated-temperature behaviour than the base 800 grade. This means the requirement sits with the heat treatment rather than with the chemistry, so a tube can meet every chemistry limit and still have inadequate creep strength if the grain size is finer than the specification requires. Grain size is verified to ASTM E112 and should appear as a result on the mill certificate.

Q4: What is the difference between 800H and 800HT?

The difference is in the aluminium and titanium contents and in the resulting creep strength. Both grades have the same base composition of nickel, chromium and iron and both are solution treated for a coarse grain size, but 800HT carries a higher combined aluminium plus titanium content, which increases the creep and stress-rupture strength at the upper end of the temperature range. The practical consequence is that 800HT can be used at a slightly higher temperature or a slightly higher stress than 800H for the same design life, or can achieve a longer life at the same conditions. It is selected where the rupture calculation for 800H leaves insufficient margin, and its cost premium is small relative to the cost of replacing a tube bundle earlier than planned. Our comparison of 800H and 800HT with Inconel 601 covers the selection in furnace service.

Q5: What does sigma phase do to an 800H tube?

It reduces ductility and toughness rather than strength, and it therefore reduces the tolerance of the tube to thermal transients and to restraint. Sigma phase is a chromium-rich intermetallic phase that precipitates in these alloys during long exposure in the intermediate temperature range, roughly 600 to 900 °C depending on composition and time, and its presence is detected by metallographic examination and often by an increase in hardness. A tube containing sigma phase can still be within its calculated rupture life and still fail in a brittle manner during a shutdown, a start-up or a process upset, because the material no longer has the ductility to accommodate the strain. This is the main reason a rupture calculation is not a complete life assessment and why metallography of a sample tube adds information that no calculation provides.

Q6: Can a bulged tube be repaired?

Generally not by welding, and the attempt is usually a false economy. A bulge indicates that the material has accumulated creep strain, which means the microstructure has changed and the remaining rupture life at that location is substantially consumed. Welding a patch onto a crept tube introduces a new heat-affected zone with its own properties, adds restraint, and is performed by a procedure that is difficult to qualify for a component already in the damaged condition. The conventional approach is to replace the affected tube, or the affected pass, and to use the measurement data to decide the extent. Where a local bulge has been caused by a local cause such as flame impingement or a restriction, the cause should be corrected at the same time, because otherwise the replacement will accumulate strain at the same rate.

Q7: How often should furnace tubes be measured?

The interval should be set by the rate of change rather than by a calendar. A practical approach is to establish a baseline at commissioning or immediately after a replacement, measure at a defined interval, and shorten the interval when the measured rate of deformation indicates that the design life will be consumed before the next planned outage. For a furnace operating at the upper end of its temperature range, an interval of one to two years is common; for a furnace with generous margin, longer intervals may be justified. The important characteristics of the programme are that the measurement positions are fixed and identified, that the technique is consistent, and that the records are kept so that rates rather than absolute values can be compared.

Q8: What causes a tube to run hotter than the design temperature?

Several things, and they are usually cumulative rather than sudden. Uneven firing across a heater produces a temperature distribution, so the tubes in one pass run hotter than the average, and burner condition, flame shape and refractory damage all contribute. Flow maldistribution between parallel passes means that some tubes carry less process flow and therefore reach a higher metal temperature at the same heat input. Internal deposits, catalyst accumulation or partial blockage have the same effect. Finally, the design metal temperature may simply have been optimistic, for example where it was based on the average gas temperature rather than on the local peak. A temperature survey is therefore part of the assessment of any creep failure, because the material condition identifies what happened while the temperature data identify why.

Q9: Is a centrifugally cast tube better than a wrought tube?

It depends on the temperature and on the fabrication requirement. Centrifugally cast tubes in HK40 or HP-modified grades offer substantially higher creep strength at the highest service temperatures, which is why they dominate pyrolysis and severe reformer duty, and their cost per unit of life can be favourable. Against that, cast tubes are less ductile, more difficult to weld, more sensitive to thermal transients, and available in a more limited range of sizes and in longer lead times. Wrought 800H and 800HT tubes offer better ductility and weldability, easier fabrication and wider availability, and for the many furnace duties below about 900 °C they are the practical choice. The decision, as always in this application, follows from the rupture calculation at the actual metal temperature rather than from a general preference.

Q10: What should I put on the purchase order for replacement tubes?

The grade with its UNS number, the product specification, the size with the minimum wall and the tolerance basis, the grain size requirement with the test method, the solution treatment requirement, the mechanical test requirements, and the documentation type. Add the requirements that the grain size result and the heat treatment record appear on the certificate, and state whether the tubes are to be supplied as straight lengths or fabricated. Where the tubes are to be welded into a heater, the welding procedure and its qualification requirements should be stated, together with the filler metal specification. Our purchase specification guide contains a fuller checklist, and most of the items on it exist because a tube that meets the chemistry specification has still failed early.

Q11: Do you supply tubes for high-temperature furnace service?

Yes, in Incoloy 800H and 800HT and in the related grades used for furnace duty, with the mill certificate carrying the chemical analysis, the mechanical test results, the grain size determination to ASTM E112 and the heat treatment records, and with document types to EN 10204 3.1 or 3.2 as the order requires. We also supply the matching filler metals and can arrange third party inspection by SGS, BV or TUV including witnessed testing. Because the creep performance of these tubes depends on the grain structure and the solution treatment rather than on the chemistry alone, our practice is to confirm the specification requirement for grain size and treatment in writing before the order is placed, so that the requirement appears on the certificate rather than being assumed.

Q12: How does the cost of the right tube compare with the cost of a failure?

Very favourably, and the comparison is worth making explicitly when a replacement is being budgeted. The premium for a tube supplied to a specification that includes the grain size and treatment requirements is a small fraction of the tube cost, and the cost of an unplanned furnace shutdown for a tube rupture includes lost production, the emergency replacement of a pass or a coil, and the inspection of everything downstream of the failure. Where a failure occurs in service, the replacement is bought under time pressure at the worst possible moment for lead time and price, and the assessment work has to be done after the event rather than before it. The material controls are therefore best treated as part of the design rather than as a procurement detail.

Conclusion and Selection Rules

Incoloy 800H fails in high-temperature service by creep rupture, and the failure is a design outcome rather than an accident. The life of the tube is decided by the metal temperature, the stress and the grain structure, and the deformation that precedes rupture is measurable throughout, which means the progression can be monitored and the replacement planned. The diagnosis of a failure and the specification of a replacement both depend on the same three pieces of information: what the metal temperature actually was, what condition the material is in, and what the design assumed.

Three rules are worth applying. Specify the grain size and the solution treatment explicitly and require the results on the certificate, because those requirements carry the creep strength that the grade is selected for. Establish a diameter baseline and measure at fixed positions, because a rate of deformation is worth more than an absolute measurement. And identify the root cause before ordering replacement tubes, because a replacement that does not address the cause will consume its life at the same rate as the tube it replaces.

Shanghai Hangbo Alloy Group Co., Ltd. supplies Incoloy 800H and 800HT in seamless tube, pipe, plate, bar and forgings to ASTM B407, B408 and B409 and the corresponding ASME designations, with chemical analysis, mechanical testing to ASTM E8/E8M, grain size determination to ASTM E112, heat treatment records, PMI to ASTM E1476, and inspection documents to EN 10204 3.1 or 3.2 with third party inspection by SGS, BV or TUV. Send your duty conditions, tube size and specification requirements through our contact page and we will confirm the grade, the grain size requirement and the documentation before quoting.

Contact & Complete Product Range

Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier

Email: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com

All Grades

Ni: NI200-N6 | Monel: 400-502, K-500, R-405 | Inconel: 600-740H, X-750, 725 | Incoloy: 800-27-7MO | Hastelloy: C-276-HYBRID-BC1, N | Super Austenitic: 904L-AL-6XN | Chromium-Rich: Alloy 33 | High-Strength Stainless: Nitronic 50/60, PH13-8Mo | Chlorination: Alloy 45 | Nimonic: 75-263 | Haynes: 25-HR-160 | RA: 330/333 | Precision: 1J22-4J45 | High-Strength: 18Ni250-AerMet100 | Heating: Cr20Ni80/60 | Specialty: Alloy 20-602CA

Product Forms: Bars(6-500mm) | Tubes(OD 6-610mm) | Plates(0.1-100mm) | Forgings | Welded Pipes | Fittings | Wire(0.05-10mm) | Strip(0.02-4.0mm)

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