Development of an efficient evaluation concept for the validation of cryotreatment of tool steels based on instrumented cyclic indentation tests

为了提高工具钢的机械性能，从而延长工具的使用寿命，可以使用低温处理，其中包括奥氏体化和淬火，低温冷却过程，温度低至-180°C以及回火。然而，为了根据所使用的材料和所考虑的应用来优化工艺参数，需要付出很大的努力，因为低温处理的影响只能在应用范围内进行评估。作为一种替代方法，低温处理钢的力学性能可以被彻底表征，但这需要很高的实验努力。因此，对工艺参数变化的评估是非常耗时的。这项研究工作的目的是开发一种概念，使用基于仪器循环压痕测试（CIT）的短时间程序PhyBaLCHT对低温处理工具的性能进行有效评级。初步工作表明，用这种方法确定的材料参数对评定低温处理工具钢的力学性能是有用的。为了获得足够的数据库，CIT将在由Vanadis 4E工具钢制成的不同低温处理的试样上进行，其中将考虑不同的工艺顺序和回火温度。除此之外，将特别关注循环低温工艺对机械性能的影响，这与单步低温工艺或常规淬火回火所达到的效果有关。在此基础上，对不同性能的低温处理Vanadis 4E的寿命和损伤机理进行了疲劳试验以及常规和绝热落料试验，并考虑了不同的加载条件。这些结果将与PhyBaLCHT确定的材料参数相关联，从而能够详细阐述评级概念，从而达到该项目的主要目标。为了确保这一概念在其他材料和工具上的可转移性，将对低温处理过程中初始状态下的微观结构及其演变进行彻底的表征。此外，还将对疲劳和落料试验中施加的不同载荷引起的微观结构变化进行分析。这一概念旨在有效评估低温处理对工具机械性能的影响，从而显著加速低温处理的发展。因此，该研究项目的目标对工具钢的低温处理领域具有重大的科学和工业意义。

To improve the mechanical properties of tool steels and thus, the lifetime of tools, a cryogenic treatment can be used, which contains an austenitizing and quenching, a cryogenic cooling process at temperatures down to -180 °C as well as a tempering. However, to optimize the process parameters with respect to the material used as well as the application considered, a high effort is required, as the influence of the cryogenic treatment can only be rated within the application. As an alternative, the mechanical properties of the cryogenically treated steel can be thoroughly characterized, which however, requires a high experimental effort. Consequently, the evaluation of a change of the process parameters is very time consuming. The objective of this research work is the development of a concept for an efficient rating of the properties of cryogenically treated tools using the short-time procedure PhyBaLCHT, which is based on instrumented cyclic indentation tests (CIT). As shown in preliminary work, the material parameters determined by this method are useful for rating the mechanical proper-ties of cryogenically treated tool steels. To obtain a sufficient data base, CIT will be performed at differently cryogenically treated specimens made of the tool steel Vanadis 4E, whereby different process sequences and tempering temperatures will be considered. In addition to that, a special focus will be put on the influence of a cyclic cryogenic process on the mechanical properties, in relation to the effects achieved by a single step cryogenic process or a conventional quenching and tempering. Based on these investigations, the lifetime as well as the damage mechanisms of different variants of cryogenically treated Vanadis 4E with sufficiently different properties will be analyzed in fatigue tests as well as conventional and adiabatic blanking tests, whereby the different loading conditions applied will be considered. These results will be correlated to the material parameters determined by PhyBaLCHT, which enables the elaboration of the rating concept and thus, reaching the main objective of this project. To ensure the transferability of this concept to other materials and tools, the microstructures in the initial states as well as their evolution during the cryogenic treatment will be characterized thoroughly. Moreover, this will be complemented by analyses of the microstructural changes caused by the different loadings applied in the fatigue and blanking tests. This concept aims at an efficient rating of the influence of a cryogenic treatment on the mechanical properties of tools and hence, a significant acceleration of the development of cryogenic treatments. Consequently, the objective of this research project is of great scientific and industrial interest for the field of cryogenic treatments of tool steels.