VDL Weweler has developed a novel heat treatment approach for their trailing arms, which eliminates the traditional austenisation step and instead capitalises on the inherent heat used for hot-forming to austenise the steel before quenching. This not only saves energy, but surprisingly also improves the lifetime of the products. The underlying reasons for this improvement in fatigue life are the central focus of the present study. Two trailing arms were used for this study, one from the traditional ‘Reference’ process, and one from the new ‘Gasloos’ process. Both were produced from the same heat of raw 51CrV4 steel bars that are inductively heated to 1200°C before hot-forming, after which Gasloos is immediately quenched to 180°C. Reference is left to cool to 600°C before being reheated in a gas oven at 900°C for one hour and is finally also quenched to 180°C. After quenching, both are tempered at 450°C for one hour, quenched to room temperature, and shot peened. Reference also underwent the usual painting procedure, Gasloos was collected before this. Both underwent fatigue bending tests, where the Gasloos trailing arm lasted about seven times longer. Fracture surfaces and unfatigued parts were collected from both trailing arms for analysis. Since it was obvious early on that the Reference trailing arm has a partially intergranular fracture, a literature survey was performed to assess the possibility that grain boundary segregation and embrittlement would occur. Scanning Electron Microscopy (SEM) combined with Energy Dispersive X-Ray Spectroscopy (EDS) was used to perform fracture and microstructural analysis, as well as to measure grain boundary segregation. X-Ray Diffraction (XRD) analysis was used to measure the depth profiles of the residual stress to assess the effectiveness of the shot-peening. Samples for XRD were cut using Electronic Discharge Machining (EDM). In both trailing arms, cracks initiated on slag inclusions present from the steelmaking process. Reference initiated on an inclusion 10μm in size, while Gasloos initiated on an inclusion 45μm size, 4.5 times bigger. Slag inclusions are on average 16.09μm in size. The Reference fracture surface is 16% intergranular, while Gasloos is fully transgranular. Carbide density on the PAGB is more than twice as high in Reference, while overall carbide density is 10% higher in Gasloos. Solute segregation to the PAGB could not be measured with EDS. XRD analysis shows that the compressive residual stress from shot-peening is 1.5 times higher and twice as deep in Gasloos compared to Reference. This is due to the oxide scales, containing 17.49 weight% oxygen, that comprise the surface of Reference, making it very brittle and filled with voids. Gasloos does not have these oxide scales at the surface.

Bainitic steels are the type of steels which are widely in use, especially in automotive industries, for its good mechanical behavior, i.e., with coexistence of strength, ductility and fatigue resistance. A better understanding on the mechanism and transformation kinetics of bainite are necessary in order to improve the performance of the bainitic steel further. This research mainly studies the kinetics of bainitic transformation during isothermal treatments. The chemical composition, in particular silicon (Si), affects the transformation kinetics of bainite, and therefore the fractions of retained austenite and bainitic ferrite. The current research focuses on the bainite transformation kinetics during isothermal heat treatment. Steel specimens are austempered at 250°C, 300°C and 350°C and held for 30, 60 and120 minutes, respectively. The steel in the current research has the alloy composition of Fe-0.61C-1.62Si-0.85Mn-0.32Cr (wt.%). Microstructures observed using optical micrographs consist of bainite and retained austenite after the austempering process. Quantitative measurements of the retained austenite (RA) fraction are performed by magnetization technique. The results show that, at austempering temperatures of 250°C and 300°C, the fraction of retained austenite decreases gradually with increasing holding times and increases with increasing austempering temperatures. However, a different affect is observed in the steel austempered at 350°C for 30minutes. The fraction of retained austenite increases from 30 to 60 minutes and subsequently decreases from 60 to 120 minutes. In order to study the effect of retained austenite on hardness resulting in decrease in overall hardness with increasing austempering temperatures and increases with increasing hold duration. The JMAK model has been fitted to the observed fraction of austenite as function of time and temperature, which results in the parameters such as the rate constant and Avrami exponent. The fitted results suggest a one-dimensional grain growth of the bainite in this steel. The predictions from a thermodynamic analysis using the para-equilibrium model and the T0-temperature are compared to the experimentally observed fractions of RA, which results in obtaining higher fractions than obtained experimentally. This thermodynamic analysis predicts an increase in the fraction of remaining austenite with increasing austempering temperature as observed experimentally in most cases. The thermodynamic study on the effect of Si concentration (1.62 wt.%)results shows that there is insufficient Si present in the steel to hinder the formation of cementite.

51CrV4 is a spring steel commonly used for the production of trailing arms in the suspension assembly of trucks. It is essential for these products to be predictable and reliable. The presence of carbide banding in 51CrV4 steel can be detrimental to this predictable and reliable behavior. Therefore, in this research, the formation of carbide-banded microstructure in 51CrV4 steel will be investigated. The bending stresses undergone by trailing arms make it crucial to understand the relationship between surface depth and carbide band formation. Therefore, the goal of this research is to understand the relation between the surface depth and band formation in 51CrV4 steel and how banding differs among 51CrV4 steel from different manufacturers. This was done using a variety of testing methods which include microscopy, hardness, XRD, SEM, EDS and EPMA. These testing methods were used to understand the formation of carbide bands in 51CrV4 as well as the underlying physical properties that lead to the carbide band formation. Overall, a significant difference could be seen in the formation of carbide bands across the steel from different manufacturers. These differences in carbide banding properties could be clearly linked to the difference in the physical properties of the steels. The depth at which carbide band formation tends to start could be most clearly related to the micro-segregation peak concentration. Furthermore, the manner in which the carbide bands are built up along the surface depth could be most clearly related to the chromium concentration in the carbide banded regions. Finally, it was found that chromium is expected to be the primary carbide former in this steel as the formation of carbide bands is closely and consistently related to an increased presence of chromium.

The presence of bainite and retained austenite in advanced high-strength steels (AHSS) has grown a great interest in the automotive industry as simultaneously provides strength and the transformation-induced plasticity (TRIP) effect. This type of microstructure is present in modern trailing arms. A trailing arm is part of the air suspension system in heavy vehicles. In spring steels, such as the ones used to produce trailing arms, this combination of bainite and retained austenite is obtained by a heat treatment named austempering. In this work, the microstructural distribution of a silicon spring steel (Fe-0.6C-1.63Si- 0.97Mn-0.48Cr) was investigated after austempering for 1 hour at 300o C. Microstruc- tural heterogeneity was investigated at different scales using optical microscopy, scan- ning electron microscopy (SEM), electron probe micro analysis (EPMA), X-ray diffraction (XRD), and hardness measurements. Observed heterogeneities were related to the pres- ence of thermal gradient, chemical segregation, and carbon gradient. The results gave insights into the effect of the processing route on the microstruc- ture. After austempering, bainitic ferrite, martensite-austenite islands, and carbide par- ticles were observed. The thermal gradient was confirmed by the identification of auto- tempered M-A islands due to the slower cooling rate in the bulk of the material. More- over, chemical segregation of Si, Mn, and Cr parallel to the rolling direction was observed and confirmed from optical microscopy, SEM, and EPMA results. In regions with high concentrations of substitutional elements, the Bs temperature was locally reduced and the bainitic ferrite formation kinetics was slow. Carbon gradient in M-A islands was ob- served as carbide precipitation occurred at the center of the island indicating that the carbon content was lower compared to at the edges of the constituent. Furthermore, in areas of the trailing arm with lower fractions of retained austenite (≺ 8%), bainite forma- tion was ceased based on the To curve (austenite reached a specific carbon content over which growth of bainite based on the diffusionless theory is not possible). These results indicate that bainitic ferrite advances non-uniformly through the trailing arm. Based on these indications, the carbon gradient had a more significant effect on the bainitic ferrite formation in the spring steel during austempering compared to the ther-mal gradient and chemical segregation.