Why does martensitic stainless steel need to be tempered twice?

Taking 9Cr18 as an example, because the carbon and chromium content in the steel are relatively high, the martensitic transition termination point MF will be reduced to a lower temperature below zero, and there will be more residual austenite after quenching. After cryogenic treatment, it can generally be transformed by about 10%, and the hardness can increase HRC1-. If there is no cryogenic treatment equipment, secondary tempering can be used instead of cryogenic treatment to eliminate more residual austenite, but the effect is not as good as cryogenic treatment.

High speed steel tempering many times is to reduce or eliminate the paralympics, martensitic stainless steel has a high CR content, tempered brittleness, it needs to be tempered quickly to eliminate tempering brittleness, and fast will produce stress, through tempering at a lower temperature again to eliminate stress Personal opinion, for reference only! View the original post

How does it work? First of all, it is necessary to understand that the martensitic stainless steel to be tempered for the second time must be used after high temperature tempering, generally speaking, 3Cr13 and 4Cr13 are mostly used after low temperature tempering (in a few cases, they are used after high temperature tempering), and only need to be tempered once. For martensitic stainless steels after high temperature tempering, in order to suppress tempered brittleness, it needs to be quickly cooled in oil or water.

This creates a certain amount of stress. In order to eliminate the stress caused by fast cooling, a stress removal tempering of about 400 degrees is supplemented. View the original post

High speed steel tempering many times is to reduce or eliminate the Paralympics, martensitic stainless steel has a high CR content, tempered brittleness, need to be tempered quickly to eliminate tempering brittleness, and fast will produce stress, through a lower temperature tempering again to eliminate stress.

View the original post

Stainless steel can be observed with a special erosion agent, but it is difficult to determine whether the tempering is sufficient. My method is to use the hardness method. Here's how:

The hardness of the tempered sample (both HV and HRC can be recorded) is recorded, and then 20 degrees lower than the original tempering temperature, and the hardness is tested after cooling for 2 hours, if the hardness changes HRC<1, or HV<10 can be considered to be fully tempered at this tempering temperature. However, the tempering furnace temperature should be accurate when verifying. This is one of my commonly used failure analysis methods.

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Answer: Martensitic stainless steel generally needs to be quenched, for example, as a tool steel, its hardness must be improved by quenching. But.

Martensitic steel is also widely used in structural steel, at this time heat treatment must choose quenching and tempering treatment (quenching + tempering) use, for martensitic tempering treatment, we must pay attention to some problems.

1. Martensitic stainless steel has tempering brittleness. Therefore, the cooling rate and tempering temperature during tempering must be selected appropriately.

2. The second type of tempering brittleness is that the impact value decreases when the tempering temperature is slowly cooled. When this type of steel is used as structural steel, great attention is paid to the impact value. Therefore, the cooling rate of tempering must be changed according to the required mechanical properties.

3. The first type of tempering brittleness depends on the tempering temperature, so it cannot be tempered within a certain temperature range. Secondly, the corrosion resistance also becomes very bad in a certain tempering temperature range, because the temperature range of the first type of tempering brittleness and corrosion resistance deterioration is roughly the same, so tempering must be avoided in this temperature range.

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