420 Stainless Steel Heat Treatment: Hardness, Tempering and Uses

Written by: Emma, Technical Sales Engineer  |  Reviewed by: Ethan, Materials Engineer  |  Updated: August 2026

420 stainless steel heat treatment is the process that unlocks this grade’s defining characteristic: the ability to reach a high hardness. 420 is a martensitic stainless steel with enough carbon to respond strongly to hardening, which is why it appears in cutlery, wear components, valves, and machined mechanical parts where hardness matters more than corrosion resistance.

This article explains why 420 can be hardened, how hardening and tempering affect its properties, and how the heat-treatment condition drives grade selection. It is written for heat-treatment engineers, machining and fabrication engineers, materials engineers, and B2B buyers.

1. Direct Answer

420 is a martensitic stainless steel. Through austenitizing, quenching, and tempering, its hardness, strength, and toughness can be adjusted over a wide range. The carbon content gives 420 a pronounced hardening response — more so than lower-carbon grades such as 410.

The trade-off is real: higher hardness generally comes with lower toughness and a certain corrosion penalty. The final choice must therefore balance hardness, toughness, the corrosion environment, the product form, and the applicable specification.

2. Quick Comparison of Heat-Treatment Conditions

This table describes purpose and general effect, but deliberately avoids absolute hardness values, which depend on the standard, product form, and section size.

3. Why 420 Can Be Hardened

The hardenability of 420 comes from its metallurgy, which is fundamentally different from austenitic stainless steels.

• Martensitic structure: 420 is designed to form martensite, a hard phase, when cooled rapidly from high temperature.
• Carbon: Carbon is the element that makes hardening possible, and 420 carries enough carbon to produce a strong hardening response.
• Chromium: Roughly 12–14% chromium provides corrosion resistance and influences transformation behavior.
• Austenitizing: Heating into the austenite range dissolves carbon into the structure.
• Quenching: Rapid cooling transforms austenite into martensite.
• Martensite formation: The result is a hard, strong, but relatively brittle structure.

This is the key difference from austenitic grades such as 304 and 316, which remain austenitic on cooling and cannot be hardened by heat treatment in the same way. Understanding this distinction avoids a common procurement error: specifying a martensitic grade like 420 when the intent was actually an austenitic, formable, non-hardenable stainless steel, or vice versa.

4. The 420 Heat Treatment Process

The main thermal processing steps are annealing, austenitizing, quenching, and tempering, each with a distinct role.

• Annealing: Softens the material for machining or forming;
• Austenitizing: Heating into the austenite range to dissolve carbon;
• Quenching: Rapid cooling to form martensite;
• Tempering: Reheating to reduce brittleness and set the final hardness;
• Cooling: The cooling rate after austenitizing controls martensite formation;
• Section thickness: Thicker sections cool more slowly and may not harden as deeply.

If specific temperature ranges are cited, their source, condition, and range of applicability must be stated. A parameter set from one product form or section size should not be treated as a universal procedure for all 420 products. In practice, the heat treater and the designer work together to confirm that the chosen cycle produces the required hardness while leaving the part in a usable, dimensionally stable condition.

5. Hardness and Tempering

Hardening and tempering work together to reach the intended property balance.

• As-quenched hardness: Immediately after quenching, 420 can reach a high hardness, but the structure is brittle;
• Tempering: Reheating relieves internal stress and trades a controlled amount of hardness for improved toughness;
• Hardness/toughness balance: The tempering step is where this balance is set;
• Tempering temperature: Higher tempering temperatures generally reduce hardness further but improve toughness;
• Tempering time: Time at temperature also affects the final properties;
• Dimensional stability: Proper tempering and stress relief contribute to dimensional stability in service.

Different temper conditions produce different mechanical properties, so it is not correct to say "higher hardness is always better." The target hardness must be matched to the actual service requirements, including toughness and dimensional behavior.

A useful way to think about the sequence is as a deliberate property trade-off. Annealing maximizes machinability, hardening maximizes hardness and strength, and tempering restores toughness at a controlled cost in hardness. The choice of temper is often the single most important decision in a 420 component, because it sets whether the part is optimized for cutting performance, wear resistance, or a balanced combination of the two.

6. Mechanical Properties

The mechanical properties of 420 are inseparable from its heat-treatment condition:

• Hardness: Ranges widely from annealed to hardened states;
• Tensile strength: Rises with hardening, falls with tempering toward the annealed condition;
• Yield strength: Similarly depends on the condition;
• Elongation: Generally higher when annealed and lower after hardening;
• Toughness: Higher after tempering than in the as-quenched state;
• Wear resistance: Generally improved by higher hardness.

7. Corrosion and Service Limits

420 has moderate corrosion resistance, and heat treatment does not turn it into a high-alloy chloride-resistant grade.

• Atmospheric exposure: Adequate in many mild, dry environments;
• Moisture: Suitable where moisture exposure is limited or controlled;
• Chloride: Limited resistance; chlorides can cause pitting;
• Pitting: Can occur where chlorides concentrate;
• Crevice corrosion: Tight gaps remain a risk.

Corrosion performance is influenced by surface finish, heat treatment, passivation, and the environment. Higher hardness does not mean higher corrosion resistance, and 420 should not be described as a 316L-level chloride-resistant stainless steel.

For a martensitic grade, the practical guidance is to keep the service expectation aligned with what the grade is built for. 420 earns its place in mechanical and wear applications where hardness is the priority and corrosion exposure is mild or controlled. Where the environment is the primary threat — chloride-laden, marine, or chemically aggressive — a higher-alloy grade is generally the better starting point, regardless of how carefully the 420 has been heat treated.

8. Applications and Selection

420 is used where hardness and wear resistance are the driving requirements:

• Cutlery;
• Surgical and instrument components where applicable;
• Valves;
• Shafts;
• Molds;
• Wear-resistant components;
• Machined and hardened mechanical parts.

Consider 420 when:

• High hardness or wear resistance is important;
• Heat treatment is part of the design;
• Moderate corrosion resistance is sufficient.

Do not use 420 directly for:

• Severe chloride environments;
• Marine exposure;
• Highly aggressive chemical service;
• Applications where corrosion resistance is the primary design requirement.

9. Standards and RFQ Requirements

When buying 420, the heat-treatment condition must be specified.

• UNS S42000: The UNS designation for 420;
• ASTM A240 / A276 where applicable: The standard must match the product form;
• Product form: Sheet, plate, bar, or rod;
• Heat-treatment condition: Annealed, hardened, or tempered as required;
• Hardness / mechanical requirements: The target hardness or strength range;
• Surface: The required surface condition;
• MTC: A material test certificate;
• PMI: Positive material identification where required;
• Testing: Any specified mechanical or inspection requirements.

An RFQ should confirm at minimum: grade, UNS designation, product form, standard, dimensions, heat-treatment condition, hardness or mechanical requirements, surface condition, and inspection requirements.

Common Purchasing Mistakes

• Treating 420 like an ordinary austenitic stainless steel;
• Chasing maximum hardness without considering toughness;
• Neglecting tempering and expecting a tough as-quenched part;
• Equating hardness with corrosion resistance;
• Mixing mechanical data from different product forms;
• Failing to confirm the heat-treatment condition;
• Using 420 directly in severe chloride service;
• Not verifying the UNS, standard, and MTC;
• Treating one heat-treater’s parameters as a universal standard.

FAQ

Q1: Can 420 stainless steel be heat treated?
Yes. 420 is a martensitic stainless steel, so it can be hardened and tempered through heat treatment, unlike austenitic grades such as 304 and 316.

Q2: How hard can 420 stainless steel become?
420 can reach a high hardness because of its carbon content, but the achievable value depends on carbon, section size, cooling rate, and tempering. No single hardness applies to every product form.

Q3: What heat treatment is used for 420 stainless steel?
The typical sequence is austenitizing, quenching to form martensite, and then tempering to set the final hardness and toughness. Annealing is used when softening is required.

Q4: Why is tempering necessary after hardening 420?
The as-quenched condition is hard but brittle. Tempering reduces brittleness and balances hardness against toughness, making the part suitable for service.

Q5: Does tempering reduce 420 stainless steel hardness?
Generally yes. Tempering trades a controlled amount of hardness for improved toughness, and the tempering temperature determines how much hardness is given up.

Q6: Is 420 stainless steel corrosion resistant?
It has moderate corrosion resistance, suitable for many mild environments, but it is not a high-alloy chloride-resistant grade and should not be used in severe chloride or marine service.

Q7: Is 420 better than 410 for hardness?
Generally yes. 420 has a higher carbon content than 410, which allows it to reach higher hardness. However, the choice should also consider toughness and corrosion requirements.

Q8: Is 420 suitable for chloride environments?
It has limited chloride resistance and is not recommended for severe chloride or marine service, where pitting can occur.

Q9: What is UNS S42000?
UNS S42000 is the UNS designation for 420 stainless steel. The applicable product standard (such as ASTM A240 or A276) depends on the product form.

Q10: What heat-treatment information should be included in a 420 stainless steel RFQ?
Include the grade, UNS designation, product form, standard, dimensions, the required heat-treatment condition, target hardness or mechanical requirements, surface condition, and any inspection requirements.

Related Reading

• 420 Stainless Steel Sheet: Grades, Condition and Thickness
• 420 Stainless Steel Plate: Heat Treatment and Hardness
• 420 Stainless Steel Coil: Finish and Tolerance
• 410 Stainless Steel Heat Treatment: Hardening, Tempering and Properties
• 416 Stainless Steel Machinability: Heat Treatment and Corrosion Limits
• Martensitic Stainless Steel Grades: Hardening and Selection

Sources and Further Reading

For authoritative standards, grade data, and heat-treatment principles, the following organizations provide technical material on stainless steel grades:

• ASTM International — https://www.astm.org/
• worldstainless — https://worldstainless.org/
• British Stainless Steel Association (BSSA) — https://bssa.org.uk/
• Nickel Institute — https://nickelinstitute.org/
• Outokumpu — https://www.outokumpu.com/
• Alleima — https://www.alleima.com/
• ISO — https://www.iso.org/