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.
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
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.