Subcritical heat treatment and machining of low-carbon steel

Abstract

In machining tests using high-speed steel tools, very large differences were observed in the upper limit to the cutting speed before complete tool failure when machining a very-low-carbon steel (0·04%C) in quench-aged, hot-rolled, and subcriticall y annealed conditions. Under standardized cutting conditions the limiting speeds were 46 m min−1 for quench-aged steel and 213 m min−1 for subcritically annealed steel. The difference is attributed to a higher stress being imposed on the main cutting edge of the tool by ferrite containing precipitated carbide or nitride particles. Large variations in limiting cutting speed were observed also when machining 0·09 and 0·15%C steels, but subcritical heat treatment had very little effect on a 0·4%C steel. The tests provided evidence that, with low-carbon steel in the normalized or hot-worked condition, precipitated particles in the primary ferrite could be responsible for large reductions in limiting cutting speed. Elimination of these particles by subcritical annealing could be responsible for important increases in limiling cutting speed and in more consistent tool performance.