420 Stainless Steel Heat Treatment Guide

To heat treat 420 stainless steel, preheat the steel to about 650°C / 1200°F, then austenitize at about 954–1010°C / 1750–1850°F, followed by air or oil quenching. After quenching, temper immediately according to the required hardness, toughness, and corrosion resistance.

For high-hardness applications, 420 stainless steel is usually tempered at lower temperatures. Typical reference points are about 150°C / 300°F for around 52 HRC minimum, 204°C / 400°F for around 50 HRC minimum, and 316°C / 600°F for around 48 HRC minimum. The tempering range of 427–566°C / 800–1050°F should generally be avoided, as it may reduce impact strength and corrosion resistance.

420 stainless steel is a hardenable martensitic stainless steel. After proper heat treatment, it typically reaches 46–52 HRC in working applications while maintaining useful wear resistance, polishability, and moderate corrosion resistance. Final performance depends on section size, quenching method, tempering temperature, furnace atmosphere, and the required application.

This guide explains the practical heat treatment process for 420 stainless steel, including preheating, austenitizing, quenching, tempering chart, expected hardness, common mistakes, and application-based heat treatment direction.

420 Stainless Steel Heat Treatment Process at a Glance The

420 stainless steel heat treatment final result depends on heating temperature, soaking time, quenching method, tempering temperature, surface protection, and part geometry.

Annealing is not part of the main hardening cycle. It is used when the material needs to be softened before machining, forming, or re-hardening. For 420 stainless steel, subcritical annealing may be carried out around 675–760°C / 1245–1400°F, while full annealing may be carried out around 830–885°C / 1525–1625°F, followed by controlled cooling.

Step-by-Step Heat Treatment of 420 Stainless Steel

The main hardening cycle for 420 stainless steel consists of three steps: preheating, austenitizing and quenching, and tempering. Cleaning and atmosphere control should be completed before heating, but the heat treatment result is mainly decided by these three stages.

Step 1: Preheat 420 Stainless Steel

Preheating reduces thermal shock before the steel reaches the austenitizing temperature. This is important because stainless steels conduct heat more slowly than many carbon and low-alloy steels. Fast heating can create temperature differences between the surface and the core, increasing the risk of distortion or cracking.

A common preheating temperature for 420 stainless steel is about 650°C (1200°F).

Before preheating, the surface should be clean. Oil, grease, and carbon-rich residues should be removed. For parts requiring good polishability or corrosion resistance, heating should be carried out in a vacuum, a neutral atmosphere, or a neutral salt bath.

Step 2: Austenitize and Quench 420 Stainless Steel

After preheating, heat 420 steel to the common hardening temperature 954–1010°C / 1750–1850°F.

The exact temperature should be selected based on section size, furnace condition, steelmaker data, and the required final hardness. The higher end of the range may be used when maximum hardness and corrosion resistance are required, but overheating should be avoided, as it can increase grain growth, distortion, and the risk of cracking.

A typical soaking guideline is about 30 minutes per inch of the greatest cross-sectional thickness. The goal is uniform heating through the section. Excessive soaking does not improve the result and may make the part less stable during quenching.

After soaking, the part should be quenched.

Air quenching is often safer for complex shapes. Oil quenching may be used when higher hardness is required, and the part geometry can tolerate a more severe quench.

Cooling must be fast enough through the critical range to avoid harmful carbide precipitation at grain boundaries. Slow cooling can reduce corrosion resistance. After quenching, the part should be tempered as soon as practical.

Step 3: Temper 420 Stainless Steel

For high hardness, wear resistance, polishability, and corrosion resistance, lower tempering temperatures are usually preferred.

Double tempering is often used for critical 420 stainless steel parts. The part should cool to room temperature between tempering cycles. This helps relieve internal stress and improve dimensional stability.

The range around 427–566°C / 800–1050°F should generally be avoided. Tempering in this range can reduce impact strength and corrosion resistance. If a softer, tougher condition is required, the tempering temperature is usually raised to about 593°C / 1100°F or higher, with a clear acceptance of lower hardness.

420 Stainless Steel Tempering Chart

The work-hardness of heat-treated 420 stainless steel is typically around 46–52 HRC. Final hardness depends on austenitizing temperature, quenching speed, tempering temperature, section size, and steel condition.

The chart below should be used as a practical reference, not as a guaranteed final hardness value.

For applications requiring hardness above 50 HRC and good corrosion resistance, low-temperature tempering is usually the safer choice. This is common for plastic molds, cutting tools, wear parts, polished components, and corrosion-exposed parts.

Common Heat Treatment Mistakes with 420 Stainless Steel

Most heat-treatment problems with 420 stainless steel stem from poor process control. The most common failures are distortion, cracking, insufficient hardness, poor surface quality, reduced corrosion resistance, and loss of toughness.

One serious mistake is using oil quenching for every part. Oil quenching can improve the hardening response, but it also increases the risk of distortion and cracking. For complex or irregular parts, air quenching is often the safer choice.

Another common mistake is using a high tempering temperature to improve toughness without considering corrosion resistance. For 420 stainless steel, high-temperature tempering can reduce hardness and may reduce corrosion performance. When high hardness and corrosion resistance are required, low-temperature tempering is usually preferred.

The most important risk zone is 427–566°C (800–1050°F). This range should not be used casually. If the part needs higher toughness, the process should be designed around 593°C / 1100°F or above, with the understanding that hardness will drop.

420 Stainless Steel Heat Treatment Direction by Application

The basic hardening process for 420 stainless steel is similar across many applications, but the tempering temperature should change according to the required performance.

For plastic molds, 420 stainless steel is often selected because it offers a combination of hardness, polishability, and corrosion resistance. In this case, low-temperature tempering is usually preferred. High-temperature tempering may still produce usable mechanical properties, but it can reduce corrosion resistance.

For blades and wear parts, hardness and edge retention are usually the main targets. Lower tempering temperatures are commonly used. If the part chips or cracks in service, the solution is not simply to increase hardness. The quenching method, tempering temperature, part design, and service load should be reviewed.

For shafts, gears, rollers, and loaded mechanical parts, maximum hardness may not be the best target. These parts may need a more balanced hardness-toughness balance. The heat treatment should match the actual load condition, not only the highest possible HRC value.

Aobo Steel supplies 420 stainless steel in an annealed condition for machining and further heat treatment by the customer or local heat treatment provider.

We do not provide final hardening or heat treatment processing. The heat treatment data in this guide is provided as a technical reference for customers, toolmakers, and heat treatment professionals.

For bulk 420 stainless steel supply, available sizes, and export details, contact [email protected].