Scania has experienced difficulties with large variations of the slope of the back plane after press quenching of case hardened crown wheels of especially type R780 Steg supplied from ingot cast material. This leads to that a large number of crown wheels has to be remeasured and sorted according to back slope which is time consuming for operators. Also, after sorting of the crown wheels, hard machining has to be adjusted according to the different slopes of the back plane of the crown wheels. In some cases, it also leads to scrapping of the crown wheels.This master’s thesis was divided in two parts. The aim of the first part was to confirm that the crown wheel type and casting technique that exhibits the largest variations in slope of the back plane is the R780 Steg originating from ingot cast material. The crown wheel types that were compared were the R780 Steg, R780 Slät and R885 Slät. Crown wheels manufactured from ingot cast material and from continuous cast material were compared. Hence, 6 combinations were examined. The slope of the back plane was measured with the measuring probe FARO after press quenching. The slope of the crown wheels was found to depend on both casting technique and the geometry of the crown wheel. The results confirmed that the crown wheel type and supplier combination that by far yields the largest variations in slope of the back plane is the R780 Steg supplied by Steel Plant A who uses the ingot casting technique. For this combination the variation exceeds 0,1 mm. All other combinations of crown wheels and suppliers yield acceptable variations.The second part of this master’s thesis was composed of determining if segregations in the cast ingot are the cause of the variations in slope of the back plane of the crown wheel type R780 Steg. This was done by measuring if there is a correlation between the slope of the back plane of the crown wheel after press quenching, the chemical composition and the original position of the crown wheel in the ingot. As in the first part of the study, the distortion was measured by the measuring probe FARO. The samples were sent to Degerfors Laboratorium for chemical analysis. Analyses of C, S and N were made by using combustion analyses. For As, P, B and Al optic spectrometry (spark) was used. All other elements were analysed by x-ray fluorescence. Segregations were found to be present and in combination with the geometry of R780 Steg to be the cause of the large variations in slope of the crown wheels.The results of this thesis show that, for the crown wheel type R780 Steg, Scania should not use suppliers that employ the ingot casting technique. Instead, only suppliers using the continuous casting technique should be used. However, for the other crown wheel types ingot or continuously cast material can be used.

Hardening distortions are unwanted changes in shape and dimension that arise during hardening of steel components. Uncontrolled distortions induce random errors to the manufacturing process, and have a strong negative impact on manufacturing costs. The distortions are not only caused by the hardening process, several factors from previous manufacturing steps including the component geometry itself contribute to varying extent. The aim of the current work is to investigate the main influencing factors on hardening distortions for serial produced gears. The investigations were done on two different types of gears for heavy-duty transmissions, crown wheels for the rear axle central gear and main shaft gears for the gearbox. The steel was produced using either continuous casting or ingot casting. For rectangular continuously cast steel, the effect of disabling magnetic stirring of the steel melt during casting was investigated, finding a strong reduction of gear runout for crown wheels. Segregations in crown wheels produced from the top and bottom of ingots were shown to go in opposite directions, producing opposite back-face tilts. For crown wheels quenched one at a time, influences of stacking level on the hardening tray were found, indicating an impact from small variations in the carburizing process, despite identical quenching conditions. For main shaft gears, horizontal loading gave considerably less roundness and runout errors but increased flatness errors compared to vertical loading. This thesis shows the complexity of the distortion phenomenon and how several factors interact and contribute to the final result. It is shown that factors with significant impact on hardening distortions for one component may be less important for another component. With this in mind, each type of component to be hardened should be produced by a manufacturing chain where each process step is carefully chosen with respect to minimizing distortions. / <p>QC 20170516</p>

During the last 10 years, low pressure carburizing and high pressure gas quenching has become more popular since it is a “new” process and researchers reports that the distortion characteristics gained from these processes is improved in relation to conventional processes. The aim of this work was to investigate the distortions gained from three different heat treatment processes on main shaft gears and crown wheels. Experiments with atmospheric carburization with oil quenching and low pressure carburization with nitrogen gas quenching or oil quenching were made and distortion characteristics gained from these processes were then measured and compared. It was found that components treaded in the relatively new LPC-HPGQ process are similar and often even better than the conventional method. The larger main shaft gear improves the most with newer method but instead gets a lower core hardness and systematic diametrical shrinkage. Some measurements on the crown wheel were also improved but others were affected negatively.