Carbon is critical to developing the correct crystalline structure. Higher carbon steels are much easier to quench and are required when the wall of the pipe is thick. Additional carbon is expensive and in most application the carbon is held intentionally low because the lean chemistry costs less per ton. A well designed quench can process lean chemistry.
Summary
- Carbon and Alloy - Enough carbon must be present to form austenite.
- The Critical cooling rate is the minimum rate of quench to produce 100% martensite.
- Alloy elements (chromium, nickel, molybdenum, manganese, silicon, and tungsten) reduce critical cooling rate by retarding formation of pearlite.