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Reasonable machining hardness range for iron and steel castings

As for machining process, the hardness is an important issue. Too high hardness will harm the cutters, and even cause the failure of machining. However, as for the cast iron and cast steel, what is the reasonable hardness range?

According to my experience in the iron foundry in China, the following hardness range is suitable.

1. The reasonable hardness range to cast iron

Normally, the hardness of cast iron is indicated by BH (Brinell Hardness) or HB (Hardness of Brinell). The hardness less than HB 220 will be suitable for normal machining process. In other words, they are suitable for normal steel alloy cutters. Most of iron foundries could machine the iron castings in this hardness range.

We suggest you to control the hardness under HB 210. Because HB 220 is still a little difficult for the drilling and tapping.

However, by the special steel alloy cutters or ceramic cutters, the cast iron with hardness under HB 300 can be machined.

2. The reasonable hardness range to cast steel

Since the cast steel is more brittle than cast iron, so it can be machined more easily.

The cast steel with hardness under HB 330 can be machined by the normal steel alloy cutters. If we transfer it into Rockwell Hardness, it is about RH 35. The cast steel with hardness under HB 500 (RH 50) can be machined by the special steel alloy cutters.

In brief:
As for Cast Iron
Under BHN 220: Normal
BHN 220 to BHN 300: Special

As for Cast Steel
Under BHN 330, HRC 35: Normal
BHN 330, HRC 35 to BHN 500, HRC 50: Special

BHN means Brinell Hardness Number. HRC means Rockwell Hardness.

The metal casting designers and foundries should consider the hardness range carefully, and arrange the production and machining process accordingly.

As for the cast steel gears, we normally make the rough casting with normal hardness, then machine and hob it, then do the heat treatment to improve its surface hardness, and finally grind it to remove the surface oxide surface.

Higher hardness will cause the more serious brittleness, so the reasonable hardness range and heat treatment process is very important for the outstanding materials.


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