420 vs 431 – Composition, Heat Treatment, Properties, and Applications

Introduction

Engineers, procurement managers, and manufacturing planners frequently confront the choice between AISI 420 and AISI 431 martensitic stainless steels when specifying parts that require a balance of strength, hardness, corrosion resistance, and cost. The selection dilemma commonly centers on trade-offs such as maximum achievable hardness and wear resistance versus toughness and corrosion resistance in aggressive environments, along with considerations for weldability and post‑processing costs.

The decisive material difference between these two grades is their alloying strategy: 420 relies primarily on higher carbon with moderate chromium for hardness and wear resistance, whereas 431 adds significant nickel plus higher chromium to improve strength, toughness, and corrosion performance while retaining martensitic hardenability. Because both are martensitic stainless steels used in similar component families (shafts, fasteners, valves, blades), engineers routinely compare them when specifying parts that must be hardened and yet resist corrosion.

1. Standards and Designations

• AISI/ASTM/UNS:
• 420: AISI 420 / UNS S42000 (often referenced in ASTM A666 for sheet/strip; ASTM A276 for bars)
• 431: AISI 431 / UNS S43100 (found in ASTM A582 for bars, ASTM A276 for bars/rods)
• EN / EN equivalents:
• 420: EN 1.4021 / X46Cr13 (similar family)
• 431: EN 1.4057 / X90CrNi18-? (exact cross-references vary by standard)
• JIS / GB: regional designations exist for martensitic stainless grades with similar chemistries.
• Classification:
• Both 420 and 431 are martensitic stainless steels. They are not classified as carbon steels, tool steels, or HSLA; they are alloyed stainless steels designed to harden by quenching and tempering.

2. Chemical Composition and Alloying Strategy

Table: typical composition (wt%) — ranges shown are representative of common specifications; check purchase spec for exact limits.

Element	420 (typical wt%)	431 (typical wt%)
C	0.15 – 0.40	0.15 – 0.25
Mn	≤ 1.0	≤ 1.0
Si	≤ 1.0	≤ 1.0
P	≤ 0.04	≤ 0.04
S	≤ 0.03	≤ 0.03
Cr	12.0 – 14.0	15.0 – 17.0
Ni	≤ 1.0 (often very low)	1.5 – 3.0
Mo	trace/none	typically none
V	trace	trace
Nb	—	—
Ti	—	—
B	—	—
N	trace	trace

Notes: - Values shown are indicative; many specifications have narrower limits. 420 is a higher‑carbon, moderate‑Cr martensitic grade; 431 is a nickel‑containing martensitic grade with higher chromium and controlled carbon to enable a better combination of toughness and corrosion resistance. - Nickel in 431 raises tensile strength and toughness and improves resistance to brittle fracture during heat treatment and service. Chromium content controls passive film formation and corrosion resistance; the higher Cr in 431 enhances general corrosion behavior relative to 420.

How alloying affects behavior: - Carbon: primary hardenability and martensite hardness; higher carbon gives higher maximum hardness but reduces toughness and weldability. - Chromium: passivation and hardenability; more Cr typically improves corrosion resistance and tempering resistance. - Nickel: stabilizes the martensitic matrix toward toughness, reduces susceptibility to temper embrittlement, and improves strength‑toughness balance. - Minor elements (Mn, Si, P, S, V) influence deoxidation, machinability, and inclusion behavior; sulfur increases machinability but can impair corrosion fatigue.

3. Microstructure and Heat Treatment Response

Typical microstructures: - Both grades form predominantly martensite after austenitizing and quenching. In the annealed condition they can contain ferrite + pearlite or tempered martensite depending on process and alloy content. - 420: after quench and temper, microstructure is martensitic with carbide precipitates (chromium carbides). Higher retained carbon leads to higher hardness but more carbides that can act as crack initiation sites if not tempered correctly. - 431: forms martensite with a finer distribution of carbides and some retained austenite depending on austenitizing/quenchant; nickel promotes a more ductile, tougher martensitic matrix.

Heat treatment response: - Normalizing: refines grain size and homogenizes microstructure; useful pre‑conditioning prior to final quench and temper. - Quenching & tempering: both grades respond well. 420 achieves high hardness (can exceed ~50 HRC in many heat treatments) but with reduced toughness at higher hardness levels. 431, with nickel and higher Cr, attains a better toughness-to-strength ratio after quench & temper and has improved resistance to tempering softening. - Thermo‑mechanical processing: benefits are mostly realized in bar/shaft products where controlled rolling prior to quenching improves grain structure; both steels can be produced with enhanced toughness through controlled processing, with 431 generally gaining more benefit because of its alloy balance.

4. Mechanical Properties

Table: typical mechanical properties (representative ranges; dependent on heat treatment and section size)

Property (typical)	420 (Q&T/tempered)	431 (Q&T/tempered)
Tensile strength (MPa)	600 – 1200	700 – 1300
Yield strength (0.2% Rp0.2, MPa)	450 – 1000	600 – 1100
Elongation (%)	8 – 20	8 – 18
Impact toughness (Charpy J)	low-to-moderate (depends on temper)	moderate-to-high (better than 420 at similar hardness)
Hardness (HRC)	Annealed ~20; Q&T up to ~55+	Annealed ~20–30; Q&T up to ~50–55

Interpretation: - 431 generally offers higher yield strength and better impact toughness at comparable tensile strength levels due to Ni and higher Cr. For the same target hardness, 431 typically shows less embrittlement and better fatigue performance. - 420 achieves high hardness and wear resistance with relatively simple chemistry and is often chosen where maximum edge retention or wear resistance is required and toughness/corrosion are secondary.

5. Weldability

Weldability considerations center on carbon equivalent and presence of alloying elements that increase hardenability and the risk of martensitic cold cracking.

Useful weldability indices: - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - International parameter $P_{cm}$: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Higher carbon and higher Cr/Mo/V raise $CE_{IIW}$ and $P_{cm}$ — increased risk of HAZ martensite formation and cold cracking. 420’s higher carbon content tends to make it more susceptible to HAZ hardening and cracking without preheat or post‑weld heat treatment. - 431’s nickel content reduces the severity of martensite hardness in the weld heat-affected zone and improves ductility; therefore 431 typically exhibits better arc weldability than 420 at similar section thicknesses and welding processes. - Practical guidance: both grades often require preheat and controlled interpass temperatures for reliable welding. Post‑weld tempering or PWHT is advisable for critical components to relieve residual stresses and reduce HAZ hardness.

6. Corrosion and Surface Protection

• Both 420 and 431 are martensitic stainless steels and are not as corrosion‑resistant as austenitic grades (e.g., 304/316). They rely on their chromium content for passive film formation.
• Use of PREN (Pitting Resistance Equivalent Number) is less common for martensitic steels but the formula is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index emphasizes Mo and N for pitting resistance; since neither 420 nor 431 contains significant Mo or N, PREN is of limited applicability.
• Practical corrosion behavior:
• 420: moderate corrosion resistance in mild environments; susceptible to pitting and crevice corrosion in chlorides and to general corrosion if not properly finished or protected.
• 431: improved general corrosion resistance over 420 due to higher chromium and nickel; widely used in seawater and petrochemical applications where better resistance to chloride stress and corrosion fatigue is required.
• Non‑stainless surface protections (for either grade when extra protection is needed): galvanizing is possible for some forms but can be limited for high‑hardness parts; painting, conversion coatings, electroplating, or passivation treatments are commonly applied depending on service.

7. Fabrication, Machinability, and Formability

• Machinability:
• 420: good machinability in annealed condition; high hardness after heat treatment reduces machinability and tool life. Sulfured variants or free‑machining subgrades may be available.
• 431: moderate machinability; tougher matrix can increase tool forces but produces good surface finish when controlled. Machining often performed in annealed or stress‑relieved condition before hardening.
• Forming and bending:
• Both are formable in annealed condition; springback and work‑hardening behavior require process control. Deep drawing is possible for thin sections in annealed condition.
• Grinding and finishing:
• 420 in hardened condition is often used for blades and wear parts; grinding and polishing are standard finishing operations.
• 431 finishes well and can be polished to a bright surface; its better toughness reduces cracking during grinding and aggressive finishing.

8. Typical Applications

420 – Typical Uses	431 – Typical Uses
Cutlery and knives (blades where edge retention is primary)	Aerospace and high‑strength fasteners (bolts, studs)
Surgical instruments and dental tools (often in hardened condition)	Pump shafts, valve components in petrochemical and marine service
Bearing cages, bushings, small wear parts	Shafts, couplings, torsion bars requiring combined corrosion and strength
Decorative trim, hardware where moderate corrosion resistance acceptable	High‑stress corrosion‑exposed components (marine bolting, hydraulic pistons)

Selection rationale: - Choose 420 for applications prioritizing wear resistance, edge retention, simple heat treatment, and lower material cost when service corrosion is limited. - Choose 431 for components that must combine high strength, toughness and improved corrosion resistance (particularly in chloride-bearing or dynamic load environments) where the incremental material cost is justified.

9. Cost and Availability

• Relative cost: 420 is typically less expensive than 431 because it contains little or no nickel; nickel is one of the more costly alloying elements. 431’s nickel and higher chromium content lead to higher raw‑material cost.
• Availability: both are common industrial grades and are widely available in bar, rod, flat, and fastener forms. Specialty sizes or surface finishes may have longer lead times, and 431 may be slightly less stocked than 420 in commodity markets.

10. Summary and Recommendation

Table summarizing key trade-offs:

Characteristic	420	431
Weldability	Moderate–challenging (high C)	Better (Ni improves toughness)
Strength–Toughness balance	High hardness, lower toughness at same hardness	Better toughness at comparable strength
Corrosion resistance	Moderate	Better (higher Cr + Ni)
Cost	Lower	Higher

Choose 420 if: - The primary requirement is high hardness and wear or edge retention at low to moderate cost. - The component operates in a relatively benign environment or will receive protective coatings. - Machining/finishing and hardening practices are well controlled and welds are minimal.

Choose 431 if: - The application demands a higher strength‑to‑toughness ratio, improved corrosion resistance (especially in chloride environments), and better resistance to temper embrittlement and fatigue. - Weldability and impact resistance are important and the project budget can accommodate higher material cost. - The part will undergo dynamic loading, seawater exposure, or requirements for higher fracture toughness.

Final note: Always verify the exact chemical and mechanical limits in your purchasing specification and select heat treatment practices (austenitizing temperature, quench medium, temper cycle, and any PWHT) to match the design requirements for hardness, toughness, and corrosion performance.