Tensile Properties and Residual Stresses of a Diesel Cylinder Head in GAS9C1 Alloy 2005-01-1692

The objective of the present paper is to report the measures we have taken to improve the tensile properties, in particular the percent elongation, of the cylinder heads in GAS9C1 alloy. These measures include the choice of modification element, the reduction of Fe content, the improvement of heat treatment parameters in order to achieve an optimum combination of strength and ductility. Simultaneously, residual stress measurements were also performed with cylinder head castings which had undergone the different cycles of heat treatment in different furnaces.

Sodium and strontium have been used for the modification of GAS9C1 alloy which was applied for the production of diesel cylinder heads. Strontium modification has increased the alloy fluidity allowing to cast cylinder heads at lower pouring temperature than using Na modification. Higher percentage elongation was obtained with Sr modification than with Na modification. Different cycles have been tried for heat treatment of cylinder heads in order to verify the feasibility of combination of high tensile strength, hardness and elongation.

Residual stresses of cylinder heads in GAS9C1 alloy have been measured by destructive cutting method. Results show that increasing the aging temperature from 180°C to 220°C (aging time 5h) decreases the residual stress of about 30 Mpa; the aging temperature increase has a more pronounced effect on reducing residual stress than prolonging aging time from 5h to 10h at 180°C. Cylinder heads quenched in hot water (>60°C) after solution treatment and aged at a lower temperature (T6 at 180°C) can have residual stress similar to those quenched with cold water (RT about 20°C) and aged at a higher temperature (T7 at 220°C). Measurements performed on castings treated in discontinuous furnaces with manual quenching show that cylinder heads with Sr modification have lower residual stress than castings with Na modification.