01-17-2018, 12:13 PM
Mark, yes exactly. If you have a look at the iron-carbon diagram on page 20, you can see on the left side near room temperature that ferrite, or alpha, can only dissolve a tiny amount of carbon, 0.02% maximum at higher temperatures. So, that carbon has to go somewhere, and it combines chemically with iron to form cementite, Fe3C, iron carbide, different names for the same thing.
Once you start to heat the steel to harden it and cross into the austenizing temperatures, the Fe3C can dissolve into the austenite and the carbon is free to move around. Now, with steels with more than about 0.8% carbon, not all the cementite dissolves at once, and you can choose, within limits, how much you want based on the temperature you heat to before quenching. It is this "undissolved" carbide that has such a large influence on wear resistance and toughness in steels. The difference in wear resistance between 1095 and 1075 at the same hardness is due to the presence of these undissolved carbides in the 1095 and not present in 1075.
Mr. Jan, you are correct. However, the difference between the density of alpha iron (ferrite) and cementite is only about 3%, so volume percent carbide and weight percent carbide are close enough for most purposes. This was a source of confusion for me when I started looking into steels, as the two were used without that explanation. It's so common that it took some real digging to find an acceptable reason to look at a weight percent graph and get volume percent carbide.
Once you start to heat the steel to harden it and cross into the austenizing temperatures, the Fe3C can dissolve into the austenite and the carbon is free to move around. Now, with steels with more than about 0.8% carbon, not all the cementite dissolves at once, and you can choose, within limits, how much you want based on the temperature you heat to before quenching. It is this "undissolved" carbide that has such a large influence on wear resistance and toughness in steels. The difference in wear resistance between 1095 and 1075 at the same hardness is due to the presence of these undissolved carbides in the 1095 and not present in 1075.
Mr. Jan, you are correct. However, the difference between the density of alpha iron (ferrite) and cementite is only about 3%, so volume percent carbide and weight percent carbide are close enough for most purposes. This was a source of confusion for me when I started looking into steels, as the two were used without that explanation. It's so common that it took some real digging to find an acceptable reason to look at a weight percent graph and get volume percent carbide.