Stainless Steel Families: Austenitic, Ferritic, Martensitic, Duplex, and Precipitation Hardening Grades
Stainless steels are iron-chromium alloys containing a minimum of approximately 10.5 wt% Cr (per EN 10088-1), sufficient to form a self-repairing chromium oxide (Cr2O3) passive film that confers corrosion resistance far beyond that of plain carbon or low-alloy steels. Within this broad family, five distinct microstructural classes — austenitic, ferritic, martensitic, duplex, and precipitation hardening (PH) — span an extraordinary range of compositions, microstructures, and engineering properties. Selecting the correct family, and then the correct grade within it, is one of the most consequential decisions in corrosion-critical or high-strength applications.
✓ Key Takeaways
- All stainless steels require ≥10.5 wt% Cr; the Cr2O3 passive film is what distinguishes them from plain carbon and low-alloy steels.
- Austenitic grades (304, 316) dominate production by volume, offering excellent corrosion resistance and formability but susceptibility to chloride stress corrosion cracking (SCC).
- Duplex grades combine austenite + ferrite (~50/50), delivering roughly double the yield strength of 304 with dramatically improved SCC resistance.
- Pitting Resistance Equivalent Number PREN = %Cr + 3.3×%Mo + 16×%N quantifies resistance to localised corrosion; PREN > 40 indicates superduplex/superaustenitic performance.
- Sensitisation (Cr23C6 precipitation at grain boundaries in the 450–850°C range) is the primary weld-related corrosion risk in standard austenitic grades; low-carbon (L) and stabilised grades (321, 347) mitigate it.
- PH grades (17-4 PH, 15-5 PH, 17-7 PH, A286) achieve yield strengths of 1000–1300 MPa through coherent nanoscale precipitates formed during ageing, while retaining stainless corrosion resistance.
The Chromium Passive Film: The Foundation of Stainless Behaviour
Stainless steels resist aqueous corrosion through the spontaneous formation of a chromium-rich oxide passive film at the metal surface. This film is amorphous to nanocrystalline, typically 1–3 nm thick, and composed primarily of Cr2O3 with contributions from FeCr2O4 spinel and hydroxide layers. Its critical property is self-repair: if mechanically disrupted in an oxidising environment, the film reforms within milliseconds to seconds.
The minimum chromium content for passivity is approximately 10.5 wt% Cr (EN 10088-1). Below this threshold, the oxide film is discontinuous and iron-rich, offering no significant corrosion protection. Above it, chromium activity at the surface is sufficient to maintain film continuity. Molybdenum (added in grades such as 316 and 2205) enriches the passive film at the metal-film interface, reducing film dissolution rates and improving resistance to localised breakdown in chloride-containing environments.
The Schaeffler-DeLong Constitution Diagram
The relative stability of austenite, ferrite, and martensite as a function of composition is quantified by chromium equivalent (Creq) and nickel equivalent (Nieq) numbers. The Schaeffler diagram, and its refinement by DeLong (which accounts for nitrogen), plots these values to predict the as-solidified microstructure — critically important in welding metallurgy.
Cr_eq = %Cr + %Mo + 1.5×%Si + 0.5×%Nb (Schaeffler)
Ni_eq = %Ni + 30×%C + 0.5×%Mn (Schaeffler)
Cr_eq = %Cr + %Mo + 1.5×%Si + 0.5×%Nb (DeLong)
Ni_eq = %Ni + 30×%C + 30×%N + 0.5×%Mn (DeLong — adds 30×N term)
WRC-1992 (weld metal prediction):
Cr_eq = %Cr + %Mo + 0.7×%Nb
Ni_eq = %Ni + 35×%C + 20×%N + 0.25×%CuAustenitic Stainless Steels
Austenitic grades account for approximately 70% of global stainless steel production by tonnage, making them the workhorse of the stainless family. They are stabilised in the FCC austenite phase at room temperature by nickel (typically 6–22 wt%) and, in modern alloys, nitrogen. The face-centred cubic structure has no ductile-to-brittle transition temperature, making austenitic stainless steels suitable for cryogenic service down to liquid helium temperatures.
Composition Ranges and Grade Classification
| Grade (UNS) | Cr (%) | Ni (%) | Mo (%) | C (max %) | N (%) | Key Feature |
|---|---|---|---|---|---|---|
| 304 (S30400) | 18–20 | 8–10.5 | — | 0.08 | — | General purpose; 18/8 base |
| 304L (S30403) | 18–20 | 8–12 | — | 0.030 | — | Low C for weld sensitisation avoidance |
| 316 (S31600) | 16–18 | 10–14 | 2–3 | 0.08 | — | 2% Mo improves pitting/crevice resistance |
| 316L (S31603) | 16–18 | 10–14 | 2–3 | 0.030 | — | Low C + Mo; weldable, marine service |
| 321 (S32100) | 17–19 | 9–12 | — | 0.08 | — | Ti-stabilised (Ti/C ≥5); prevents sensitisation |
| 347 (S34700) | 17–19 | 9–13 | — | 0.08 | — | Nb-stabilised (Nb/C ≥10); petrochemical |
| 310S (S31008) | 24–26 | 19–22 | — | 0.08 | — | High Cr+Ni; heat/oxidation resistance to 1100°C |
| 254 SMO (S31254) | 20 | 18 | 6 | 0.020 | 0.18–0.22 | Superaustenitic; PREN ∼43; seawater service |
| AL-6XN (N08367) | 20–22 | 23–25 | 6–7 | 0.030 | 0.18–0.25 | Superaustenitic; PREN ∼45; offshore/desalination |
Mechanical Properties
Solution-annealed austenitic stainless steels have relatively low yield strength (200–310 MPa) because the FCC lattice accommodates dislocation slip easily. However, they exhibit exceptional work-hardening capacity — the strain-hardening exponent n is typically 0.35–0.55 — enabling deep drawing and cold forming to achieve final UTS values of 700–1000 MPa. This work hardening is partly due to strain-induced martensite transformation (SIMT), where the metastable austenite transforms to α′ martensite under mechanical deformation, a phenomenon quantified by the Md30 temperature.
Md₃₀ = 413 − 462×(C+N) − 9.2×Si − 8.1×Mn − 13.7×Cr − 9.5×Ni − 18.5×Mo [°C]
Md₃₀: temperature at which 50% martensite forms after 30% true strain
Grades with Md₃₀ > 20°C are prone to SIMT at room temperature
304 typical Md₃₀ ≈ +30°C; 316 Md₃₀ ≈ −30°C (more stable austenite)Sensitisation and Weld Decay
The primary metallurgical risk in welding standard austenitic grades (304, 316) is sensitisation: precipitation of chromium carbide Cr23C6 at grain boundaries during exposure to the 450–850°C temperature range. The carbide nucleates heterogeneously on grain boundaries, consuming chromium from the adjacent matrix and depleting a narrow zone below the ~10.5 wt% Cr passivity threshold. This chromium-depleted zone is selectively attacked in corrosive service, causing intergranular corrosion (IGC) or intergranular stress corrosion cracking (IGSCC). The practical solutions are:
- Low-carbon grades (304L, 316L): C ≤0.030 wt% reduces the driving force for carbide nucleation and extends the time to sensitisation by orders of magnitude at HAZ temperatures.
- Stabilised grades (321, 347): Titanium (321) or niobium (347) are stronger carbide formers than chromium and preferentially combine with carbon, leaving insufficient carbon to form Cr23C6. The stabilisation ratios are Ti/C ≥5 (by weight) and Nb/C ≥10.
- Solution annealing: Post-weld heat treatment at 1050–1120°C dissolves Cr23C6 and restores the chromium-depleted zones, but is often impractical for large fabrications.
Stress Corrosion Cracking of Austenitic Grades
Austenitic stainless steels are highly susceptible to chloride-induced stress corrosion cracking (Cl-SCC), particularly at temperatures above approximately 50°C in the presence of tensile stress and chloride ions. The mechanism involves anodic dissolution at the crack tip, facilitated by passivity breakdown at strained regions ahead of the crack. Alloys with higher Ni content (316 vs 304) show slightly improved SCC resistance, but resistance only becomes truly robust at Ni ≥45% (Incoloy 825, alloy 625 territory). The practical engineering solution for aggressive chloride service is to switch to duplex grades or titanium alloys.
Ferritic Stainless Steels
Ferritic stainless steels have a body-centred cubic (BCC) crystal structure at all temperatures from room temperature to solidus, stabilised by chromium (10.5–30 wt%) with low carbon and nickel contents. They are ferromagnetic, have lower nickel content than austenitcs (reducing alloy cost), and offer good oxidation resistance. Their main limitations are limited toughness at low temperatures and a susceptibility to 475°C embrittlement and sigma-phase formation at elevated temperatures.
Grade Classification and Composition
| Grade (UNS) | Cr (%) | Mo (%) | Ti/Nb | Key Application |
|---|---|---|---|---|
| 409 (S40900) | 10.5–11.7 | — | Ti-stabilised | Automotive exhaust systems; lowest cost SS |
| 430 (S43000) | 16–18 | — | — | Appliances, kitchen equipment, mild corrosion |
| 439 (S43035) | 17.5–18.5 | — | Ti, low C+N | Automotive; improved weldability over 430 |
| 444 (S44400) | 17.5–19.5 | 1.75–2.5 | Ti+Nb | Water heaters, domestic plumbing; Cl resistance |
| 446 (S44600) | 23–27 | — | — | High-temperature service; furnace parts to 1175°C |
| 29-4C (S44735) | 28–30 | 3.6–4.2 | Ti+Nb, ULC | Superferritic; PREN >45; seawater condensers |
475°C Embrittlement and Sigma Phase
Two thermally induced embrittlement mechanisms limit elevated-temperature use of ferritic grades. 475°C embrittlement (sometimes called “chrome lock”) occurs at 350–525°C by spinodal decomposition: the single-phase ferritic BCC structure decomposes into Cr-rich (α′) and Fe-rich (α) domains. These coherent domains act as obstacles to dislocation motion, raising hardness and embrittling the steel. The reaction is thermally reversible by brief annealing above 600°C.
Sigma phase (σ), an FeCr intermetallic, forms in high-chromium grades (≥17% Cr) at 600–900°C. It is extremely brittle (hardness >900 HV) and substantially depletes the matrix of Cr and Mo. Sigma forms fastest around 850°C and is dissolved by re-solution annealing above 1050°C. Both phenomena impose strict thermal limits on process design for high-Cr ferritic grades.
Weldability of Ferritic Grades
Ferritic stainless steels are prone to grain growth in the HAZ because they do not undergo a phase transformation on heating — unlike carbon steels, where austenite formation limits grain growth. HAZ grain growth above ~900°C is essentially irreversible. The large columnar grains reduce toughness and ductility. Low-interstitial (“superferritic”) grades with C+N <150 ppm and Ti/Nb stabilisation maintain adequate HAZ toughness. Thin sections (<3 mm) of standard grades such as 430 can be welded without serious degradation.
Martensitic Stainless Steels
Martensitic stainless steels exploit the austenite-to-martensite transformation to achieve high hardness (up to 60 HRC in 440C) and high strength. Chromium content is 11–18 wt% and carbon ranges from 0.1 wt% (soft grades such as 410) to 1.2 wt% (wear-resistant 440C). The elevated carbon provides high as-quenched hardness but also requires careful control of heat treatment to achieve the required hardness–toughness balance through tempering.
Grade Classification
| Grade (UNS) | Cr (%) | C (%) | HRC (Q&T) | Application |
|---|---|---|---|---|
| 410 (S41000) | 11.5–13.5 | 0.08–0.15 | 25–39 | Steam turbine blades, valves, pump shafts |
| 410S (S41008) | 11.5–13.5 | ≤0.08 | 18–25 | Low-carbon; weld liners, more formable |
| 420 (S42000) | 12–14 | 0.15–0.40 | 45–52 | Cutlery, surgical instruments, hand tools |
| 431 (S43100) | 15–17 | ≤0.20 | 30–40 | Ni-bearing; marine hardware, fasteners |
| 440A (S44002) | 16–18 | 0.60–0.75 | 54–57 | Cutlery, bearings — moderate toughness |
| 440C (S44004) | 16–18 | 0.95–1.20 | 57–60 | Bearings, valve seats, knives — highest hardness |
| CA6NM (J91540) | 11.5–14 | ≤0.06 | 28–35 | Hydroturbine castings; low C, 4% Ni |
Heat Treatment of Martensitic Stainless Steels
The standard heat treatment cycle is: austenitise at 980–1070°C (dissolving carbides into austenite), oil or air quench (depending on section thickness and grade), then temper immediately at 150–750°C depending on target hardness. A critical nuance specific to martensitic stainless steels is that tempering at 400–600°C produces temper embrittlement in some grades due to chromium carbide precipitation at prior austenite grain boundaries, and is generally avoided. For maximum toughness at high strength, a “double temper” treatment is sometimes used.
Martensite start temperature (Ms):
Ms [°C] = 1302 − 42.0×%Cr − 61.0×%Ni − 33.0×%Mn − 28.0×%Si
− 1667×(%C + %N) [Kung & Rayment, 1982]
For 410 (12% Cr, 0.12% C): Ms ≈ 1302 − 42×12 − 1667×0.12 ≈ 494°C
For 440C (17% Cr, 1.0% C): Ms ≈ 1302 − 42×17 − 1667×1.0 ≈ −79°C
→ 440C requires refrigeration or sub-zero treatment to complete transformationDuplex Stainless Steels
Duplex stainless steels consist of approximately equal proportions of austenite (γ) and ferrite (α) — typically 45–55% of each — achieved through balanced composition design and a high-temperature annealing treatment (1020–1100°C) that produces the target phase ratio. They combine the SCC resistance of ferrite with the toughness and formability contribution of austenite, and their yield strength (450–700 MPa in lean to superduplex grades) roughly doubles that of standard austenitic grades.
Composition Design: Nitrogen as an Austenite Stabiliser
Unlike the austenitic family, where nickel is the primary austenite stabiliser, duplex grades rely heavily on nitrogen (0.10–0.30 wt%) because nitrogen is both a potent austenite stabiliser and a major contributor to PREN (coefficient of 16 in the PREN formula). Nitrogen raises the yield strength through solid solution hardening of both phases and markedly improves pitting resistance without requiring expensive additional molybdenum. The ferrite is stabilised by chromium (22–28 wt%), molybdenum (1–4 wt%), and silicon; the austenite is stabilised by nickel (3.5–7 wt%), manganese, and nitrogen.
Duplex Grade Families
| Category | Grade | Cr (%) | Ni (%) | Mo (%) | N (%) | PREN | Rp0.2 (MPa) |
|---|---|---|---|---|---|---|---|
| Lean duplex | LDX 2101 (S32101) | 21–22 | 1.35–1.7 | 0.1–0.8 | 0.20–0.25 | ~26 | ≥450 |
| Lean duplex | 2304 (S32304) | 21.5–24.5 | 3.0–5.5 | 0.05–0.6 | 0.05–0.20 | ~26 | ≥400 |
| Standard duplex | 2205 (S32205) | 21–23 | 4.5–6.5 | 3.0–3.5 | 0.14–0.20 | ~35 | ≥450 |
| Superduplex | 2507 (S32750) | 24–26 | 6–8 | 3–5 | 0.24–0.32 | ~43 | ≥550 |
| Superduplex | Zeron 100 (S32760) | 24–26 | 6–8 | 3–4 | 0.20–0.30 | ~41 | ≥550 |
| Hyperduplex | SAF 3207 HD (S33207) | 31–33 | 6–9 | 3–4 | 0.40–0.60 | >50 | ≥700 |
PREN (Pitting Resistance Equivalent Number):
PREN = %Cr + 3.3×%Mo + 16×%N [standard; for austenitic and duplex]
PREN = %Cr + 3.3×(%Mo + 0.5×%W) + 16×%N [PREW — includes tungsten]
Critical pitting temperature (CPT) in 3.5% NaCl solution correlates approximately:
CPT [°C] ≈ PREN − 18 (empirical, for standard duplex grades)
Grade 2205: PREN ≈ 22 + 3.3×3.2 + 16×0.17 ≈ 35 → CPT ≈ 17°C
Grade 2507: PREN ≈ 25 + 3.3×4.0 + 16×0.27 ≈ 43 → CPT ≈ 25°CIntermetallic Phase Precipitation in Duplex Steels
The high alloy content of duplex grades makes them susceptible to several intermetallic phase precipitation reactions that are more rapid than in ferritic grades of comparable composition. The ferrite phase acts as the nucleation and growth medium for these reactions:
- Sigma phase (σ): Forms at 600–1000°C; peak rate at ~850°C. In 2205, detectable sigma forms after only 1–2 minutes at peak temperature. Sigma severely embrittles and reduces corrosion resistance.
- Chi phase (χ): Mo-enriched carbide/intermetallic forming at 700–900°C; less common than sigma but similarly harmful.
- Alpha-prime (α′) and 475°C embrittlement: As in ferritic grades, affects the ferritic phase component at 350–525°C.
- Secondary austenite (γ2): Forms within the ferrite at 600–800°C, depleted in N and Mo relative to primary austenite, reducing corrosion resistance.
These precipitation reactions impose a strict thermal window for fabrication: duplex steels must be processed above ~1000°C and cooled rapidly through the embrittlement range. Post-weld heat treatment is rarely used; instead, solution annealing followed by water quenching restores the optimum duplex microstructure after welding of heavy sections.
Precipitation Hardening (PH) Stainless Steels
Precipitation hardening stainless steels combine the corrosion resistance of stainless steel with ultra-high strength achieved through age-hardening — the same strengthening principle used in aluminium alloys and nickel-base superalloys. They are designed for applications demanding tensile strengths of 1000–1400 MPa where standard austenitic or martensitic stainless grades cannot meet both strength and corrosion requirements simultaneously. Key industries include aerospace, defence, nuclear, and high-performance mechanical engineering.
Sub-family Classification
PH stainless steels are classified by the matrix microstructure in the solution-annealed condition:
Martensitic PH
Solution anneal produces austenite; on cooling, transforms to low-carbon martensite (Ms ~130–180°C). Age at 480–620°C. Examples: 17-4 PH, 15-5 PH, 13-8 PH.
Semi-Austenitic PH
Solution anneal gives metastable austenite at room temperature. Requires conditioning (austenite conditioning + refrigeration or mechanical treatment) to transform to martensite before ageing. Examples: 17-7 PH, PH 15-7 Mo.
Austenitic PH
Stable FCC matrix at all temperatures; strengthened by ageing but Rp0.2 is lower than martensitic PH. Example: A-286 (Fe–25Ni–15Cr, used in gas turbine discs).
Strengthening Precipitates in Key PH Grades
| Grade (UNS) | Matrix | Precipitate | Ageing Temp | Rp0.2 (MPa) | Typical Application |
|---|---|---|---|---|---|
| 17-4 PH (S17400) | Martensite | Cu-rich (ε-Cu), NbC | 480–620°C | 725–1170 | Aerospace forgings, pump shafts, valve stems |
| 15-5 PH (S15500) | Martensite | Cu-rich (ε-Cu), NbC | 480–620°C | 790–1170 | Heavy-section aerospace; better through-thickness toughness than 17-4 PH |
| 13-8 Mo (S13800) | Martensite | NiAl (β′) | 510–565°C | 1000–1310 | High-strength fasteners, aircraft components, gas turbine parts |
| 17-7 PH (S17700) | Semi-austenitic | NiAl | 480–565°C | 1035–1310 | Springs, diaphragms, bellows — where formability pre-ageing is needed |
| A-286 (S66286) | Austenitic | Ni3(Al,Ti) γ′ | 720°C (16 h) | 590–795 | Gas turbine discs, bolting; service to 700°C |
Condition Designations for 17-4 PH
The heat treatment conditions for 17-4 PH are standardised under ASTM A693/A693M, with designations of the form H9xx, where the number indicates the ageing temperature in degrees Fahrenheit. The H900 condition (ageing at 480°C / 900°F) provides the highest strength but lowest toughness; higher ageing temperatures provide progressively lower strength with higher toughness and corrosion resistance.
| Condition | Ageing Temp | Ageing Time | UTS (MPa) | Rp0.2 (MPa) | Charpy (J) |
|---|---|---|---|---|---|
| H900 | 480°C | 1 h | 1310 | 1170 | 14–27 |
| H925 | 496°C | 4 h | 1170 | 1000 | 27–41 |
| H1025 | 552°C | 4 h | 1070 | 1000 | 47–81 |
| H1075 | 579°C | 4 h | 1000 | 862 | 68–115 |
| H1100 | 593°C | 4 h | 930 | 793 | 81–136 |
| H1150 | 621°C | 4 h | 862 | 725 | 95–163 |
Stainless Steel Family Selection Guide
Systematic material selection requires matching the property profile of each family against the service requirements. The decision tree for stainless steel selection typically follows: (1) corrosion environment severity, (2) mechanical property requirements, (3) fabrication route constraints, (4) temperature range, (5) cost envelope. The following table provides a first-level comparison across families.
| Property | Austenitic | Ferritic | Martensitic | Duplex | PH Grades |
|---|---|---|---|---|---|
| Yield strength (ann.) | 200–310 MPa | 240–400 MPa | 275–700 MPa | 450–700 MPa | 725–1310 MPa |
| Cryogenic service | Excellent | Poor (DBTT) | Poor | Moderate (to −50°C) | Moderate |
| High-temp (>600°C) | Good (310S) | Good (446) | Poor | Not suitable | A-286 to 700°C |
| Chloride pitting | Moderate (316) | Moderate (444) | Poor | Excellent (2205) | Moderate |
| SCC resistance | Poor | Excellent | Moderate | Excellent | Good–moderate |
| Weldability | Excellent | Good (thin) | Fair (preheat req.) | Good | Fair–Good |
| Magnetic | No (mostly) | Yes | Yes | Partially | Yes (most) |
| Work hardening | Very high | Moderate | Low (as-tempered) | Moderate | Low |
| Relative cost | Moderate | Low | Low–moderate | Moderate–high | High |
Weldability of Stainless Steel Families
Welding stainless steels requires understanding family-specific transformation behaviour and thermal sensitivity. Austenitic grades are generally the most weldable — no preheat required, good dilution tolerance, and low DBTT weld metal. However, they require low heat input to minimise sensitisation risk and distortion from high thermal expansion (coefficient ~16×10-6/°C, vs ~11×10-6/°C for carbon steel). Duplex grades require heat input control to maintain the 40–60% phase balance; too-low heat input favours ferrite (reducing toughness and corrosion resistance in the weld), while too-high heat input risks intermetallic precipitation on cooling. The HAZ microstructure in duplex welds is briefly 100% ferritic at peak temperatures above ~1300°C, with austenite re-forming on cooling from approximately 1000°C downwards.
Filler metal overalloying in nickel is standard practice for duplex welding: the weld pool is richer in Ni than the base metal to compensate for the shorter time available for austenite reformation in the rapidly cooled weld metal. Over-alloyed filler metals (e.g. AWS ER2209 for 2205 base metal, ER2594 for 2507) raise the Nieq to ensure adequate austenite content in the weld.
Industrial Applications by Family
Oil and Gas
Duplex and superduplex grades dominate offshore and subsea applications: production risers, flowlines, manifolds, heat exchangers (2205, 2507). NACE MR0175/ISO 15156 governs material selection for sour service, limiting hardness in martensitic and duplex grades (22 HRC max for most applications) to prevent sulphide stress cracking (SSC). Superaustenitic grades (254 SMO, AL-6XN) are used for seawater injection systems and produced water handling.
Chemical and Petrochemical
Austenitic grades (304L, 316L, 321, 347) dominate storage vessels, heat exchangers, and piping in the chemical process industry. The selection between 304L and 316L typically depends on whether the process stream contains chlorides. Stabilised grades 321 and 347 are specified for equipment operating continuously at elevated temperature (350–850°C) where sensitisation of standard grades would occur.
Aerospace and Defence
PH grades (17-4 PH, 15-5 PH, 13-8 Mo) are extensively used for airframe fittings, landing gear components, rotor shafts, and high-strength fasteners. Austenitic A-286 and austenitic grades (321, 347) are used for exhaust systems and higher-temperature components. Titanium alloys and nickel superalloys supplement stainless for the most demanding thermal environments.
Architecture and Consumer Products
Grade 316 and 316L are the standard for architectural cladding, facades, handrails, and marine architectural applications. Grade 430 (ferritic) is cost-effective for interior applications such as appliances, kitchen equipment, and automotive trim where chloride exposure is limited. Grade 304 remains the default for food processing equipment, pharmaceutical vessels, and brewery tanks.
Energy and Power Generation
Steam turbine blades and stator vanes use 410, 420, and 17-4 PH depending on the temperature stage. High-pressure steam pipe uses 321H (high-carbon 321) for creep strength at elevated temperature. PWHT of martensitic stainless steel weldments in turbine casings requires careful documentation under applicable codes (ASME B31.1, EN 13480).
Frequently Asked Questions
What minimum chromium content is required for a steel to be classified as stainless?
Why are austenitic stainless steels non-magnetic?
What is sensitisation and how does it affect stainless steel?
What is the Pitting Resistance Equivalent Number (PREN) and what value indicates good pitting resistance?
Why are duplex stainless steels more resistant to stress corrosion cracking than austenitic grades?
What is sigma phase and why is it harmful?
Can martensitic stainless steels be welded?
How do precipitation hardening stainless steels achieve their high strength?
What is 475°C embrittlement in ferritic stainless steels?
What is the difference between 17-4 PH and 15-5 PH stainless steels?
For the related phase transformation context, see the overview of the iron-carbon phase diagram and martensite formation in steel. The role of alloying elements in controlling microstructure is also central to understanding bainite microstructure and pearlite colony growth. Corrosion-resistant selection for specific environments should be complemented by studying corrosion mechanisms and pitting corrosion in depth. Welding process inputs are covered in the HAZ microstructure article, and hardness verification methods are detailed in the hardness testing guide.
Recommended Reference Books
Further Reading
