Volume 79, Issue 2

In addition to various advantages over oil quenching, quenching in aqueous polymer solutions can also have disadvantages. Under certain conditions, “explosive” phenomena can occur when quenching steel components in such solutions, leading to pressure waves and vibrations in the quenching tank, which can even destroy the weld seams of the tank in the long term. After these processes were analyzed in the past when quenching cylindrical shafts, the processes occurring with horizontally charged disks have now been investigated. The main difference between these two geometries lies in the size of the lower disk end face, which significantly impedes the outflow of evaporated portions of the aqueous polymer solutions and thus has a pronounced influence on the cooling behavior of disk-shaped components. The investigations to be presented here were carried out in a laboratory quenching tank. The type and concentration of the polymer, the flow type and the diameter of the inner bore of the disks were varied. Temperature and conductivity were measured and audio and video recordings were made to characterize the resulting cooling processes. Selected quenching variants are presented in detail in the article. Furthermore, key figures for characterizing the various cooling variants are compared and discussed.

This report investigates the impact of different heat treatment parameters and varying Al-contents on the microstructure of Quenching & Partitioning (Q&P) steels. Therefore, three lean-medium-Mn-steels with Al-contents between 0.3 and 0.9 wt-% underwent heat treatments according to Quenching & Partitioning regimes. For comparison, the steels were subjected to a transformation induced plasticity (TRIP) aided-bainitic-ferrite (TBF) process. In both cases, the samples were fully austenitized at 900 °C for 120 s, using dilatometry. For the Quenching & Partitioning process, the quenching temperature (T Q ) ranged from 210 °C to 330 °C, while the transformation induced plasticity (TRIP) aided-bainitic-ferrite samples were cooled to 360 °C. Afterwards, the specimen were re-heated to the partitioning temperature (T P ) of 400 °C and isothermally held for partitioning times (t p ) of 40, 120 and 200 s. Subsequently, these steels were analyzed with regard to their phase fractions and hardness. The results indicated that in the Quenching & Partitioning process, the microstructure was primarily influenced by the partitioning temperature (T Q ), while partitioning times (t p ) played a minor role due to the time-independent martensitic transformation during quenching. In general, rising quenching temperature (T Q ) led to an increase in retained austenite (RA) fraction. In the transformation induced plasticity (TRIP) aided-bainitic-ferrite (TBF) process, a substantial influence of partitioning times (t p ) was found, which can be explained by the kinetics of the isothermal bainitic transformation. Regardless of the heat treatment concept, an increasing Al-content contributed to elevated retained austenite contents.

Can residual compressive stress, which is etablished using mobile processing methods without the use of specially designed equipment (mobile blasting systems or special hammering equipment), i. e., without noteworthy machine investment, significantly increase the fatigue strength of welded structural steel structures? To answer this question, samples were welded, ground and/or brushed, residual stress states were determined and fatigue tests were carried out. The results obtained show why welded constructions often are characterized by a very different service life. Currently, however, the last operation is not yet used in a standardized, targeted and prescribed manner to ensure a defined and repeatable residual stress state on the surface and thus guarantee a superior fatigue strength of welded assemblies.