Effect of stress relief annealing time on mechanical properties and operating costs in lamellar graphite cast ıron engine blocks

In this thesis study, the effects of 10-hours and 15-hours stress-relief heat treatments applied to engine blocks were investigated in terms of dimensional measurements, chemical analysis, microstructure, mechanical properties, and residual stresses. For dimensional measurement, a TR20 CMM device was used; for chemical analysis, an ARL 4460 optical emission spectrometer; for microstructure analysis, a Nikon optical microscope; for hardness testing, a Reicherter device with a 3-ton load capacity; for tensile testing, a Shimadzu tensile tester with a 250 kN capacity and extensometer; and for residual stress measurements, the strain gauge method via cutting/machining was applied. As a result of the dimensional measurements, it was determined that there were no significant dimensional differences in most regions of the parts subjected to different heat treatment recipes, with only minimal differences observed in a few areas. The chemical analysis results were in compliance with EN GJL-250 standards. Microstructural examinations revealed the presence of Type A graphite in both heat treatment conditions and a matrix structure consisting of approximately 99% pearlite. It was observed that the 15-hour annealing process resulted in longer, thinner, and more homogeneously distributed graphite particles, thereby improving the matrix–graphite compatibility and enhancing the ductility of the material.Tensile test results indicated that the 15-hour stress relief process slightly increased the yield strength and elongation at break, while having minimal effect on the ultimate tensile strength. Hardness measurements showed a slight decrease after the 15-hour process, which can be considered a beneficial effect in terms of improved machinability. Residual stress measurements demonstrated that the 15-hour stress relief heat treatment reduced residual stresses by approximately 40% to 80% in regions with initially high stress levels. This reduction contributes to lowering the risk of cracking and deformation in the engine block, thereby enhancing its thermomechanical durability and service life. The findings confirm that extending the annealing duration is an effective method for relieving internal stresses. According to a detailed analysis, increasing the heat treatment duration from 10 to 15 hours results in an additional annual cost of approximately €26,273, of which 64% (€16,949) is due to natural gas consumption, 32% (€8,400) to labor costs, and only 4% (€924) to electricity expenses. Considering the furnace capacity, approximately 10–11 tons of engine blocks are processed per annealing cycle. Based on an annual production volume of 28,000 units, this additional cost corresponds to only a 0.4% increase per part. This cost distribution highlights the importance of energy and labor optimization; however, when evaluated together with the structural improvements achieved, the cost increase is deemed acceptable.