Managing High-Temperature Carburization in Hydrocarbon Processing: A Comparative Study of Nickel-Iron-Chromium Alloys
Date: 2026年9月8日 Categories: News Views: 295
Excerpt:
Carburization - the inward diffusion of carbon and precipitation of chromium-rich carbides at grain boundaries - is the dominant life limiter for furnace hardware in ethylene pyrolysis, steam reforming and refinery hydrocarbon service at 800-1100 degC. This comparative study of Incoloy 800H, Alloy 330, Inconel 601 and Inconel 602 CA reviews the carbon-transport mechanism, explains how nickel, silicon, chromium and aluminum block or slow carbon ingress, and provides tabulated attack-rate comparisons plus a practical alloy-selection framework.
Engineering summary. Carburization — the inward diffusion of carbon and precipitation of chromium-rich carbides at grain boundaries — is the dominant life limiter for furnace hardware in ethylene pyrolysis, steam reforming and refinery hydrocarbon service at 800–1100 °C. It does not thin the wall like oxidation; it turns ductile austenite into a hard, brittle, crack-prone shell. This article reviews the transport mechanism, explains precisely how nickel, silicon, chromium and aluminum block or slow carbon ingress, and compares the four wrought Ni-Fe-Cr alloys most frequently specified for carburizing duty — Incoloy 800H (N08810), Alloy 330 (N08330), Inconel 601 (N06601) and Inconel 602 CA (N06025) — with tabulated attack-rate comparisons and a practical selection framework.
1. Why Carburization Is the Quiet Killer of Furnace Hardware
Oxidation is visible: a scale grows, spalls and tells the inspector the component is being consumed. Carburization is invisible until it cracks. Atomic carbon from the process gas dissolves into the alloy surface, migrates inward — preferentially along grain boundaries — and precipitates as chromium-rich carbides of the M₇C₃ and M₂₃C₆ type. Three consequences follow, in order of severity:
- Chromium is sequestered. The chromium that should be available to re-form the protective Cr₂O₃ scale is locked into carbide particles, so the alloy loses its environmental defense exactly where it is most needed.
- The matrix embrittles. Carburized zones are hard (frequently > 400 HV against ~180 HV parent metal) and nearly ductility-free; thermal cycling then initiates cracks that propagate through the brittle layer and into the sound metal.
- Physical-property mismatch grows. The carburized layer expands and develops a different thermal-expansion response than the core, generating high interfacial stresses on every cycle.
The result is a component that fails by cracking — often after a decoking cycle or a start-up — with little warning and little measurable wall loss. In ethylene pyrolysis coils, steam-reformer tubes, transfer-line hardware, radiant baffles, muffles, hangers and heat-treatment fixtures, carburization is routinely cited as the first or second ranking life limiter.
| Failure attribute | Oxidation | Carburization |
|---|---|---|
| Mechanism | Metal consumed by scale growth (metal loss) | Carbon absorbed into metal (mass gain, embrittlement) |
| Primary damage location | External surface | Sub-surface and grain boundaries |
| Typical temperature window | Any, accelerating > 900 °C | 700 – 1100 °C (fastest ~850 – 1050 °C) |
| Driving variable | O₂ partial pressure | Carbon activity (a_C) of the gas |
| Detection | Visual scale, thickness loss | Metallographic depth profile, hardness traverse |
| End-of-life signature | Thinning, holes | Cracking, no significant thinning |
2. The Mechanism: How Carbon Enters the Metal
Carbon transfer from a hydrocarbon process gas to a metal surface proceeds in four steps, any of which can be rate-limiting:
| Step | Transport process | Practical lever |
|---|---|---|
| 1 | Gas-phase species (CH₄, CO, C₃H₆, H₂) diffuse to the surface through the boundary layer | Gas velocity, tube/component geometry |
| 2 | Surface reactions release atomic carbon: CH₄ → C + 2H₂; 2CO → C + CO₂ (Boudouard); CO + H₂ → C + H₂O | Temperature, gas composition, catalysts (soot, Fe-Ni particles) |
| 3 | Carbon diffuses inward through the alloy — fast along grain boundaries, slower through the lattice | Temperature; alloy diffusivity; barrier scales |
| 4 | Carbon exceeds local solubility and precipitates as carbides, or is blocked by a protective oxide/silica layer | Alloy Cr, Si, Al content; pre-oxidation; oxide integrity |
The thermodynamic driver is the carbon activity (a_C) of the gas relative to the alloy. When gas a_C exceeds the equilibrium activity of the alloy's own carbon-plus-carbide system, carbon is absorbed; the higher the temperature, the faster the kinetics. Two further subtleties dominate real furnaces:
- Coking couples with carburization. In pyrolysis coils, coke deposits form on the metal, and the coke-metal interface sustains a very high local a_C. Coke spallation and decoking (steam/air) also strip the protective oxide, so carburization advances fastest where decoking has been aggressive.
- Oxidation and carburization compete for the surface. A dense, adherent oxide blocks carbon ingress; a spalled or reduced oxide opens the door. This is why alloys that form slow, adherent, self-healing scales (alumina formers with reactive-element additions) are systematically better in carburizing service than alloys whose chromia scale is spallation-prone.
| Atmosphere/condition typical of hydrocarbon furnaces | Typical a_C effect | Material implication |
|---|---|---|
| CH₄-rich pyrolysis gas, 950 – 1150 °C | High a_C at metal surface (exacerbated under coke) | Carburization dominates; scale integrity critical |
| Steam-reformer syngas (CH₄/H₂O/CO/CO₂/H₂), 850 – 950 °C | Moderate-to-high a_C | Carburization + oxidation in balance; metal dusting risk on cooling |
| H₂-rich, low-carbon gas | Low a_C | Oxidation/H₂ attack dominates, not carburization |
| Decoking transients (steam/air), 900 – 1050 °C | Scale stripped; metal briefly unprotected | Re-oxidation cycle defines long-term scale quality |
3. Alloy Design: What Blocks Carbon
Four alloying elements do the heavy lifting in Ni-Fe-Cr alloys, and a fifth (carbon itself) is a two-edged sword. Understanding their individual roles is the key to reading any carburization data table.
| Element | Mechanism against carburization | Effective range / notes |
|---|---|---|
| Nickel | Stabilizes FCC austenite; suppresses the bcc/ferrite paths that transport carbon faster; lowers carbon solubility and diffusivity in the matrix; resists sigma-phase embrittlement | The single most important element: higher Ni = lower carbon uptake rate at equal Cr |
| Chromium | Forms the external Cr₂O₃ scale (with enough O₂) that physically blocks carbon ingress; when overridden, ties up incoming carbon as Cr carbides — slowing deeper penetration but embrittling the boundary zone | 19–26 % typical in this family; needs oxide integrity to help |
| Silicon | Strongly reduces carbon activity in the matrix and oxidizes to a thin SiO₂-rich layer beneath the external scale that is an exceptional carbon diffusion barrier | Most effective at ~1–2.5 %; high-Si cast grades (HP-Modified) rely on it deliberately |
| Aluminum | Forms Al₂O₃ sub-scale — the most impermeable practical barrier to both oxygen and carbon at > 1000 °C | Requires ≥ ~1.8 % Al plus scale adhesion (Y, Zr) to be continuous |
| Carbon (in the alloy) | Raises matrix carbon activity toward gas a_C, reducing the driving force for further uptake; fine carbides strengthen matrix | 602 CA's deliberate 0.15–0.25 % C exploits this |
| Manganese | Marginally lowers carbon activity | Secondary effect only |
Nickel and silicon deserve emphasis because they act below the surface, independent of scale condition. Nickel's role is the reason Fe-Ni-Cr alloys with 32–35 % Ni (800H, 330) outperform 18/8 and 25/20 stainless in the same furnace, and why the highest-Ni wrought grades (601 at ~60 %, 602 CA at ~63 %) sit at the top of the chart. Silicon is the reason Alloy 330 — no higher in nickel than 800H but carrying ~1–1.5 % more silicon — historically outlasts 800H in heavily carburizing fixture service, and why HP-Modified cast tubing is specified with deliberate silicon bumps.
4. The Four Candidate Alloys: Composition and Positioning
| Element (wt %) | Incoloy 800H (N08810) | Alloy 330 / RA330 (N08330) | Inconel 601 (N06601) | Inconel 602 CA (N06025) |
|---|---|---|---|---|
| Nickel | 30 – 35 | 34 – 37 | 58 – 63 | 60 – 66 |
| Chromium | 19 – 23 | 17 – 20 | 21 – 25 | 24 – 26 |
| Iron | ~39.5 min / bal. | Balance | Balance | 8 – 11 |
| Silicon | ≤ 1.0 | 1.0 – 2.0 (typ. ~1.5) | ≤ 0.5 | ≤ 0.5 |
| Aluminum | 0.15 – 0.60 | — | 1.0 – 1.7 | 1.8 – 2.4 |
| Titanium | 0.15 – 0.60 | — | ≤ 0.5 (residual control) | ≤ 0.2 |
| Carbon | 0.05 – 0.10 | ≤ 0.08 | ≤ 0.10 | 0.15 – 0.25 |
| Manganese | ≤ 1.5 | ≤ 2.0 | ≤ 1.0 | ≤ 0.5 |
| Reactive elements | — | — | — | Y 0.01 – 0.10; Zr ≤ 0.10 |
| Primary defense strategy | Cr₂O₃ scale + austenite stability | Cr₂O₃ scale + Si barrier + austenite stability | Cr₂O₃ with Al₂O₃ contribution | Al₂O₃ + Cr₂O₃ duplex, Y-pegged |
| Continuous-service ceiling (oxidation) | ~1050 °C (design) | ~1150 °C (light duty) | ~1150 – 1180 °C | 1200 °C |
| Typical product forms | Bar, tube, plate, sheet, forging | Bar, plate, sheet, tube, wire mesh | Bar, tube, plate, sheet, wire | Bar, tube, plate, sheet, wire, forging |
Positioning in one sentence each:
- Incoloy 800H is the cost baseline — a creep-grade Fe-Ni-Cr austenite whose 100,000 h creep design and chromia defense are entirely adequate to ~950–1000 °C and marginal above.
- Alloy 330 is the carburizing-service workhorse of heat treatment — same Ni class as 800H but with a silicon boost and higher practical ceiling in carbon-rich, mildly oxidizing atmospheres.
- Inconel 601 upgrades the oxidation defense with 60 % Ni plus aluminum, giving the best clean-air cyclic oxidation behavior of the mid-price group, and genuinely better carburization resistance than 800H/330 when the atmosphere is strongly oxidizing enough to keep its scale intact.
- Inconel 602 CA is the top of the wrought chart: highest Cr of the group, alumina duplex scale, yttrium pegging for cycling, deliberate carbon for matrix strengthening and carburization tolerance, and valid data to 1200 °C.
Detailed composition and certification data for the two extremes of the comparison are in Hangbo's Alloy 602 CA guide and Incoloy 800H guide; Inconel 601 sits between them in both price and performance.
5. Carbide Precipitation and Microstructural Damage
Carburization damage is not uniform carbon solution — it is a microstructural cascade. As the carbon front advances, a sequence of precipitates forms, each with distinct consequences. The table below is the metallographer's map of a carburized section (from surface inward, at typical service temperature).
| Precipitate / phase | Where it forms | Temperature window | Effect on properties | Reversibility |
|---|---|---|---|---|
| M₇C₃ (Cr-rich) | Grain boundaries near surface | 700 – 1000 °C | Embrittlement, Cr depletion of adjacent matrix | Not reversible in service |
| M₂₃C₆ (Cr-rich) | Grain boundaries and intragranular, deeper front | 650 – 1000 °C | Hardness rise, ductility loss, creep-ductility fall | Not reversible |
| M₃C / cementite-type | Very high a_C surface layer, Fe-rich alloys | 850 – 1050 °C | Severe hardening, cracking under cycling | Not reversible |
| σ (sigma) phase | Long-exposure Fe-Ni-Cr with residual ferrite or high Cr/Fe ratio | 650 – 900 °C | Severe room-temperature embrittlement | Not reversible; avoid via Ni content |
| Oxide sub-scale (internal oxidation) | Grain boundaries beneath external scale | > 900 °C | Notch effect, ties up Cr/Al | Not reversible |
| Carbide-coarsened zone | Re-solutioning on over-temperature excursions | > 1050 °C (local) | Loss of fine-carbide strengthening | Partial, on re-anneal only |
Two practical rules follow. First, once carburized, always carburized: the Cr-rich carbides are thermodynamically stable in service, so weld-repairing or "burning out" a carburized zone is generally futile — the common industry guidance is that a heavily carburized furnace tube or fixture is at end of life, since embrittlement is through-wall. Second, the rate-limiting step engineers can actually control is scale quality, not gas chemistry: alloys whose oxide stays dense and adherent keep the carbon front at the surface, where it forms a removable carburized skin rather than a through-wall embrittlement.
| Alloy | Carbide-precipitation response in carburizing gas | Failure signature in service |
|---|---|---|
| Incoloy 800H | Cr-rich boundary carbides at moderate depth; sigma risk only at very long exposures | Surface cracking, loss of ductility, eventual through-wall crack at bend or weld |
| Alloy 330 | Similar carbides but Si slows front; higher Ni content delays sigma | Hard carburized skin; cracking delayed vs. 800H in same duty |
| Inconel 601 | Higher Ni lowers carbon uptake rate; Al contributes internal oxide barrier | Better skin integrity; fails by scale/cycling rather than deep carburization |
| Inconel 602 CA | Carbon front largely blocked at scale; matrix C already high, reducing driving force | Longest life; failure typically thermal-fatigue, not carburization |
6. Comparative Attack-Rate Data in Carburizing Atmospheres
No single number captures "carburization resistance" because uptake depends on temperature, gas a_C, gas velocity, cycling and prior surface condition. The tables below therefore present indicative ranges from comparative laboratory and furnace-exposure testing — use them for ranking alloys, not for predicting absolute life, and confirm against your specific gas analysis.
6.1 Carburization mass gain (1000 h exposure)
| Alloy | 900 °C, a_C ≈ 0.6 (mg/cm²) | 1000 °C, a_C ≈ 0.6 (mg/cm²) | 1100 °C, a_C ≈ 0.8, 5 % CH₄/H₂ (mg/cm²) |
|---|---|---|---|
| Incoloy 800H | ~1 – 3 | ~3 – 8 | 5 – 15 |
| Alloy 330 | ~1 – 2.5 | ~2 – 6 | ~4 – 10 |
| Inconel 601 | ~0.5 – 2 | ~2 – 5 | 5 – 15 |
| Inconel 602 CA | < 0.5 | < 0.8 | < 1 |
Ranges are indicative (manufacturer literature and published comparative tests; 800H/601/602 CA at 1100 °C per Hangbo's carburization reference data). The ranking is stable across temperatures: 602 CA > 601 ≥ 330 > 800H in net mass gain, with the 602 CA advantage largest — one to two orders of magnitude — at 1100 °C where the alumina sub-scale becomes fully effective.
6.2 Carburized-affected depth (indicative, mm per year at a_C ≈ 0.6–0.8)
| Alloy | ~900 °C | ~1000 °C | ~1100 °C |
|---|---|---|---|
| Incoloy 800H | ~0.5 – 1.5 | ~1.5 – 4 | 3 – 8 (scale spallation accelerates ingress) |
| Alloy 330 | ~0.4 – 1.2 | ~1 – 3 | ~2.5 – 6 |
| Inconel 601 | ~0.3 – 1.0 | ~0.8 – 2.5 | ~1.5 – 4 |
| Inconel 602 CA | ~0.1 – 0.4 | ~0.3 – 1.0 | ~0.5 – 1.5 |
Typical end-of-life acceptance in furnace practice is a carburized depth of a few millimetres measured metallographically on a cut sample. The table shows why 800H hardware in a 1000 °C carburizing service can reach that limit in 1–3 years, while 602 CA in the same service is measured in many years.
6.3 Metal dusting (low-temperature carburization catastrophe)
Below ~800 °C — during start-up, turndown or in waste-heat sections — the danger shifts from high-temperature carburization to metal dusting: supersaturation of carbon leads to disintegration of the metal into graphite and fine metal particles. Chromia formers are notoriously vulnerable where the scale cannot self-heal in the low-O₂ gas; alumina-forming alloys and high-Si alloys resist dusting far better.
| Alloy | Metal-dusting resistance (350 – 750 °C, high a_C) | Practical guidance |
|---|---|---|
| Incoloy 800H | Poor – moderate | Avoid in dusting-prone cool zones without surface protection |
| Alloy 330 | Moderate | Si helps; still susceptible below ~600 °C |
| Inconel 601 | Moderate | Better than 800H; not a dusting alloy |
| Inconel 602 CA | Good | Alumina scale resists the reduction/dissolution cycle that causes dusting |
7. Selection Framework: Matching Alloy to Duty
Material selection in carburizing hydrocarbon service is a four-variable optimization — temperature, carbon activity, cycling severity and price. The decision matrix below encodes standard practice for wrought Ni-Fe-Cr alloys.
| Duty scenario | Metal temperature | Recommended wrought grade | Rationale |
|---|---|---|---|
| Pyrolysis/reformer radiant hardware (hangers, supports), oxidizing + carburizing | 950 – 1050 °C | Incoloy 800H / 800HT | Adequate creep and carburization defense at the low end; lowest cost |
| Same, hotter or more carburizing | 1000 – 1150 °C | Inconel 601 | Higher Ni + Al improves scale integrity and carbon barrier |
| Severe carburizing, cyclic (fixtures, muffles, baskets) | 1000 – 1150 °C | Alloy 330 (moderate) or 602 CA (severe) | 330's Si for carbon-blocking; 602 CA when creep + cycling + carburization all peak |
| Hottest carburizing duty, long campaign, thin sections | 1100 – 1200 °C | Inconel 602 CA | Only wrought grade with valid data at 1200 °C and <1 mg/cm² uptake at 1100 °C |
| Cool zones, waste-heat, dusting risk | 350 – 750 °C | 602 CA or diffusion-aluminized lower grade | Alumina scale + surface treatment against metal dusting |
Cross-checks from the companion guides reinforce this logic: Hangbo's furnace-tube selection guide covers the cast HP-Modified and wrought coil options in pyrolysis; the heat-treatment fixture guide applies the same matrix to baskets, trays and muffles; and the general carburization guide covers inspection limits and life-extension practice.
8. Engineering Measures That Extend Life — Regardless of Alloy
Alloy choice sets the ceiling; operating and design practice decides whether you reach it. The highest-leverage measures, ranked by field experience:
- Control the carbon activity, not just the temperature — steam-to-hydrocarbon ratio, dilution and firing balance all move a_C.
- Manage the cycle. Every thermal cycle that spalls the scale hands the carburizing gas an unprotected surface. Pre-oxidize new hardware (e.g., 950–1000 °C in air) to establish the protective scale before process gas is introduced.
- Design out stress raisers and stagnant zones where coke and dusting can concentrate attack — radius edges, avoid crevices, drain low spots.
- Inspect with depth, not just thickness. Metallographic carburization-depth measurement on a cut coupon (typical acceptance: a few millimetres) plus a hardness traverse gives earlier warning than wall-thickness ultrasonics.
- Do not weld-repair carburized zones. Because the embrittlement is through-wall and carbides are stable, replace rather than repair at the turnaround.
- Decoke gently and infrequently. Steam/air decoking strips the scale; each decoke consumes a fraction of the alloy's oxidation reserve.
9. Conclusion and Supply Notes
Carburization resistance in Ni-Fe-Cr alloys is a chemistry problem with a scale-management answer. Nickel sets the baseline by stabilizing austenite and slowing carbon transport; silicon adds a sub-scale diffusion barrier; chromium provides the external scale when the gas allows it; and aluminum — in a reactive-element-stabilized duplex scale, as in 602 CA — provides the only defense that survives both 1200 °C and thermal cycling. For ranking purposes the four alloys of this study fall in a stable order — 602 CA > Inconel 601 ≈ Alloy 330 (duty-dependent) > Incoloy 800H — with the gaps widening exactly where the service is hardest: high temperature, high a_C and frequent cycling.
| Criterion | 800H | 330 | 601 | 602 CA |
|---|---|---|---|---|
| Relative carburization resistance (1100 °C) | Baseline (1×) | ~1.3× | ~1.5× | > 10× (mass gain) |
| Oxidation ceiling (cyclic) | ~1050 °C | ~1100 °C | ~1150 °C | 1200 °C |
| Relative material cost index | 1.0 | ~1.1 – 1.3 | ~1.5 – 1.8 | ~1.8 – 2.3 |
| Best-value band | ≤ ~1000 °C, mild | Carburizing, < 1100 °C, low load | Clean + mildly carburizing, cyclic | > 1050 °C, carburizing, cyclic, load-bearing |
Shanghai Hangbo Alloy Group manufactures and stocks all four grades — 800H, Alloy 330, Inconel 601 and Inconel 602 CA — in round bar, plate, sheet, seamless tube and welding wire, with EN 10204 3.1 mill certification, OES/PMI verification and third-party inspection available. For a grade recommendation tied to your furnace's gas analysis, operating temperature and cycle count, contact our technical sales team — or browse the full nickel-alloy knowledge center for the supporting data behind this comparison.
Technical FAQ
Q1. What is carburization and why is it worse than oxidation in furnaces?
Carburization is the inward diffusion of carbon from a high-carbon-activity gas into hot metal, where it precipitates chromium-rich carbides (M₇C₃, M₂₃C₆) at grain boundaries. Oxidation removes metal by scale growth and is visible; carburization adds carbon below the surface, embrittles the metal and causes cracking with little visible warning and no significant wall thinning. In ethylene and reformer furnaces, carburization is usually the life-limiting damage mode.
Q2. Why does higher nickel content improve carburization resistance?
Nickel stabilizes the FCC austenite, lowers carbon solubility and diffusivity in the matrix, and suppresses formation of sigma phase and other embrittling phases during long exposures. That is why 30–35 % Ni alloys (800H, 330) outperform 25/20 stainless, and why 58–63 % Ni grades (601, 602 CA) outperform them again. Nickel's benefit is subsurface and independent of scale condition.
Q3. How does silicon block carbon?
Silicon strongly reduces the activity of carbon dissolved in the matrix — so less carbon is absorbed at a given gas a_C — and it oxidizes to form a thin SiO₂-rich layer beneath the external scale that acts as an exceptionally effective carbon diffusion barrier. This is why Alloy 330 (~1–1.5 % Si) and high-silicon HP-Modified cast tubing are specified for heavily carburizing service.
Q4. Why does high chromium not fully protect an alloy from carburization?
Chromium's protective role depends on maintaining a dense external Cr₂O₃ scale, which requires sufficient oxygen in the gas. When carbon penetrates instead, chromium precipitates as carbides — this actually slows deeper ingress by tying up the carbon, but embrittles the boundary zone and depletes the chromium reservoir needed for scale re-formation. More chromium (24–26 % in 602 CA) delays this, but scale integrity and a second barrier (Al₂O₃, SiO₂) are what truly stop the front.
Q5. Which of the four alloys — 800H, 330, 601, 602 CA — resists carburization best?
For wrought product, Inconel 602 CA leads by a wide margin: comparative testing shows <1 mg/cm² carburization mass gain after 1000 h at 1100 °C in 5 % CH₄/H₂ versus 5–15 mg/cm² for 800H and 601, with 330 between them. The alumina duplex scale, yttrium adhesion and deliberate matrix carbon (0.15–0.25 %) combine to block carbon and reduce the thermodynamic driving force for uptake.
Q6. What temperature range is carburization most aggressive?
Damage is fastest roughly between 850 and 1050 °C, where carbon diffusivity is high and oxide scales are least protective; significant attack occurs from ~700 °C up to ~1100 °C. Below ~700–750 °C the separate failure mode of metal dusting takes over in high-a_C gases, while above ~1100 °C oxidation and scale spallation increasingly compete with carburization for control of component life.
Q7. Is Incoloy 800H acceptable in carburizing furnace service?
Yes, within its envelope. 800H is a standard, cost-effective choice for pyrolysis and reformer hardware up to roughly 950–1000 °C in moderately carburizing gas, where its chromia scale and austenite stability give decades of documented service. Above ~1050 °C, or under severe cycling or high a_C, its chromia scale spalls, carbon ingress accelerates, and a higher grade (601, 330 or 602 CA) is justified.
Q8. Can carburized components be repaired by welding or heat treatment?
Practically no. Chromium carbides formed during carburization are stable at service temperatures, the embrittlement is typically through-wall, and heat treatment cannot dissolve the damage without destroying the component's properties. Industry practice is to replace heavily carburized tubes, fixtures and muffles at the turnaround; reclaimed or weld-repaired carburized hardware fails early.
Q9. How is carburization detected and measured?
The standard method is metallographic examination of a cut coupon: the carburized zone appears as a hard, etch-different, carbide-rich layer, and its depth is measured from surface to the carbide front. A hardness traverse corroborates the boundary. Typical acceptance limits are on the order of a few millimetres of affected depth, depending on wall thickness and code. Rising measured depth between turnarounds is an end-of-life signal.
Q10. What is metal dusting and how does it relate to carburization?
Metal dusting is catastrophic low-temperature (roughly 350–750 °C) carburization in which carbon supersaturation disintegrates the metal into graphite and fine metal particles — "dust." It attacks where high-a_C gas contacts metal whose protective scale cannot form or self-heal, such as start-up, turndown and waste-heat sections. Alumina-forming alloys such as 602 CA and diffusion-aluminized surfaces resist dusting far better than plain chromia formers like 800H.
