scope:

This standard covers the requirements for induction hardening and tempering heat treatment of the cylinder bores of gray iron Duramax cylinder blocks. This standard only covers the metallurgical (microstructure and hardness) requirements of the Duramax block and the methods to verify.

Material Description. Induction hardening of the cylinder bores in the DMax cylinder block is performed to improve the wear-resistance in the piston-ring upper ring reversal zone by selectively hardening only the region necessary to accomplish improved wear resistance, see Figure 1. The choice of induction hardening for this task was due to its flexibility in heating only the precise area required and its very short cycle time. Additionally, the martensitic transformation results in an approximately 4.3% increase in specific volume, thus the pattern was developed to minimize the impact on cylinder bore distortion. Induction hardening of ferrous materials entails three basic steps; heating to the austenitizing temperature; holding to fully convert the structure to austenite; and rapidly cooling to convert the austenite to martensite. In addition, many parts are subsequently tempered to improve toughness.

Symbols. Not applicable. 

Applicability. Response to induction hardening is strongly influenced by initial process design parameters in addition to the following variables: 

• Incoming Material: composition, microstructure, residual stress. 
• Part Dimensions: bore wall variation: thickness, diameter, cylindricity.
• Process Control: coil-to-part clearance, power input, timing, quench water. 
• Documentation: hardened structure pattern, case depth.

This specification is limited to assessing and verifying the effectiveness of the hardening/tempering process in producing the microstructure, hardness and pattern of transformed microstructure on the bore inner circumference. 

Remarks. Appendix A contains the description of the incoming block material that has been found to satisfactorily respond to induction hardening, including composition, microstructure and hardness.