PSI - Issue 23

Ulrich Krupp et al. / Procedia Structural Integrity 23 (2019) 517–522 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

519

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contain the individual martensite lathes (cf. Fig. 1). In most cases, quenching is followed by a tempering heat treatment, allowing the supersaturated C to precipitate as Fe,Cr carbides. The size of the carbides and the residual martensite tetragonality determines the strength of the steel. However, the fatigue behavior is also affected by the prior austenite grain size. The higher concentration of impurities within coarse-grained prior austenite boundaries leads to the risk of intergranular failure; while in general, coarse microstructures results in more pronounced stress concentrations due to dislocation pile ups (according to the Hall-Petch mechanism). It is the objective of the current work to identify the relationship between the microstructure and VHCF strength of tempered steel (Krupp et al. (2017)) and to implement this quantitatively in a physically based short crack model based on the boundary element approach (cf. Kübbeler et al. (2011) . The VHCF damage model shall contribute to an improved implementation of non-metallic inclusions to the dimensioning of VHCF-loaded components.

2. Experimental

The experimental work of this study was performed on the two tempered steels 50CrMo4 and 16MnCrV7 7 (German designation) with the chemical composition as given in Table 1, and in various thermomechanical heat treatment conditions.

Table 1. Chemical composition of the studied materials (in wt%). material C Cr Mo

Mn

P

S

V/NbAl

50CrMo4

0.48 0,16

1.00

0.18

0.71

0.013 0,013

0.010 0,017

--

16MnCrV 7 7

1,7

--

1,7

0,17

The steel 50CrMo4 was normalized at 860°C (30 min.) and tempered, either at 200°C or at 550°C (90 min.), leading to a Rockwell hardness of 57HRC or 37HRC, respectively (cf. Krupp et al. (2017)). In the case of the steel 16MnCrV7 7 (developed for forging and case hardening), cylindrical segments of 50mm diameter were thermomechanically treated using a 10 MN screw press at 950°C und 1250°C , followed by defined quenching in air and air/water spray mixtures (cf. Fig. 2a). While water spray cooling (t 8/5 =56s) leads to martensitic transformation the subsequent decrease in the cooling rate (switch to air cooling at T=260°C) allows for martensite self annealing with fine dispersed cementite precipitates (cf. Krupp et al. (2018)).

a b Fig. 2. (a) Thermomechanical processing by means of two different cooling routes in a schematic TTT diagram for the steel 16MnCrV 7 7 applied to hot-forged bars, and (b) resonance testing machine with high-resolution thermo camera for in-situ VHCF-damage monitoring. The cyclic deformation behavior of the materials was characterized by means of resonance testing (Rumul Testronic, 100kN, max. frequency 400Hz) and ultrasonic testing (type BOKU Vienna, max. 10kN, max. frequency