Stainless steel for knives | Knife steel - Heat treatment (hardening)

Heat treatment (hardening) of stainless steel for knives

Hardening is a way to make knife steel harder. If you first heat knife stainless steel to a certain temperature (depending on the grade of steel) and then cool (harden) it quickly, the knife steel will become much harder, but also more brittle.

Heat treatment, namely tempering, plays an important role in giving a stainless steel knife the desired properties such as hardness, strength, flexibility and corrosion resistance.

The process of hardening stainless steel for knives consists of several stages:

1. Heating:

• Stainless steel is heated to a certain temperature, which is usually in the range of 980 - 1150 °C, depending on the specific type of steel.
• This temperature is above the critical point at which the austenitic structure of the steel turns into a martensitic one.

2. Soaking:

• The steel is held at this high temperature for a certain amount of time to ensure complete structural transformation.
• Soaking time may vary depending on the size and thickness of the blade..

3. Cooling:

• Rapid cooling of the blade is a key step in the hardening process.
• Usually the blade is immersed in a bath of oil or water.
• Rapid cooling "freezes" the martensitic structure, which leads to a significant increase in hardness and strength.

4. Tempering:

• Tempering is an additional heat treatment process that is performed after quenching to relieve internal stresses and improve the toughness of the stainless steel.
• The blade is heated to a lower temperature (usually 300 - 500 °C) for a certain amount of time and then slowly cooled.

To reduce brittleness, the material is tempered, typically by heating it to 175 - 350 °C for 2 hours, resulting in a hardness of 53 - 63 HRC (depending on the stainless steel grade selected) and a good balance between sharpness retention and grinding ability and strength.

Tempering should be done within a reasonable time after curing, preferably within an hour or so. It is very important that the blade cools to room temperature before tempering begins. Otherwise, the transformation to martensite will be interrupted and the hardening results may be degraded.

A higher tempering temperature will result in a slightly softer material with a higher viscosity, while a lower tempering temperature will produce a harder and slightly more brittle material.

For example, a camping or survival knife can be hardened at 350 °C to withstand rough handling and not break. On the other hand, if the knife is expected to have a sharp edge, it can be hardened at 175 °C to achieve maximum hardness.

Tempering temperatures below 175 °C should only be used in exceptional cases where extreme demands are placed on high hardness, as very low tempering temperatures will result in a very brittle material. Tempering temperatures above 350 °C should also be avoided, as this can lead to brittleness and reduced corrosion resistance. Please note that if a hardened blade is exposed to temperatures above the hardening temperature (e.g. grinding, polishing), the properties of the knife will also be impaired.

It is important to note that the exact parameters of the heat treatment, such as temperature, holding time and cooling rate, must be carefully selected for the specific type of stainless steel from which the knife blade is made.

Proper tempering will result in a good balance between hardness, strength and corrosion resistance in the finished knife blade.

Improper heat treatment can result in the blade being too brittle and prone to chipping, or too soft and not holding its sharpness.

What is the best stainless steel for knives ?

Unfortunately, today, there is no single answer to the question of the best stainless steel for knives, because it depends on your needs and preferences. Different types of stainless steel have different characteristics, so it's important to choose the one that best suits how you will use the knife and under what conditions.

Here are some factors to consider when choosing a knife steel:

• Hardness: the hardness of stainless steel determines how resistant it is to abrasion and chipping. Harder steel will hold an edge longer, but it will also be more difficult to sharpen.
• Wear resistance: is the ability of stainless steel to resist damage caused by both abrasive and adhesive wear. Abrasive wear occurs when harder particles pass through a softer surface. Adhesive wear occurs when debris is displaced from one surface and attached to another. Wear resistance is usually correlated with the hardness of the steel, but is also highly dependent on the specific chemical composition of the stainless steel. In steels of equal hardness, the steel with larger carbides (microscopic, hard, wear-resistant particles) usually resists wear better. However, carbides can become brittle and crack, resulting in reduced strength. Molybdenum and manganese: the blade is stronger and more resistant to abrasion.
• Elasticity: the elasticity of stainless steel is characterized by two main moduli: Young's modulus (modulus of elasticity of the first kind) is a measure of how much the steel can stretch or compress under load. It is measured in megapascals (MPa). Poisson's ratio is a measure of how much the steel contracts in cross-section when it is stretched or compressed in the longitudinal direction.
• Toughness: the toughness of a steel determines how resistant it is to breaking or cracking. Tougher steel will be less prone to damage from impacts or falls.
• Corrosion resistance: some types of stainless steel are more resistant to corrosion than others. This is important to consider if you will be using the knife in wet conditions. Chromium in stainless steel increases corrosion resistance.
• Edge retention: this is how long the stainless steel will hold its edge. It depends on the hardness, viscosity and chemical composition of the steel. Carbon in stainless steel for knives: The blade is more likely to be hardened and therefore get and keep a good cutting edge.
• Ease of sharpening: some types of stainless steel are easier to sharpen than others. This is important to consider if you do not have experience in sharpening knives.

The best stainless steel knife is a properly tempered knife that has a balanced chemical composition, hardness, strength, elasticity, corrosion resistance, and the ability to keep the blade sharp for a long time.