PSI - Issue 7

A. Giertler et al. / Procedia Structural Integrity 7 (2017) 321–326

322

2

A. Giertler et Al./ Structural Integrity Procedia 00 (2017) 000–000

1. Introduction Structurally loaded components in transportation, power generation and mechanical engineering are often exposed to high stress amplitudes for more than of 10 7 cycles (VHCF, very high cycle fatigue). Often, the simple approach of defining a fatigue limit for a given material does not lead to success due to possibly required lightweight or tolerance limitations. Another reason for the development of new strategies for fatigue-resistant design is the large scatter in the lifetime data in the VHCF regime. In future fatigue-design concepts, the material microstructure has to be taken into account to ensure maximum material utilization at maximum component safety. The heterogeneous microstructure itself, but also nonmetallic inclusions, pores and oxides, are of major influence on the scatter in the VHCF life, cf. Kunio et al. (1981). Tempered martensitic steels, as subject of the present study, are cost efficient materials for applications where high number of load cycles are necessary. They combine good machinability with good mechanical properties. The mechanical properties of martensitic steels can be adjusted by a two-stage heat treatment. Therefore, the material is heated until it is fully austenitic and then quenched in oil. The rapid cooling from high temperature leads to the diffusionless martensite formation, following the Kurdjumov-Sachs relationship between the fcc austenite and the bct martensite laths, Kurdjumov and Sachs (1930). The martensitic formation is following a hierarchical setup, where martensite needles are aligned within parallel blocks. Several blocks of the same orientation form a packet and several packets are located within a prior austenite grain, Morito 2006, Kitahara 2006. The strength of the material can be influenced by changing the temperature of the tempering treatment. An increase in temperature leads to a decrease of strength caused by a decrease of the tetragonal distortion of the bct lattice due to the supersaturation of the carbon and to an increase of the size of the carbide precipitates. The VHCF fatigue behavior can be divided into a type I and a type II mechanism, Mughrabi 2006. Type I behavior describes the formation and growth of persistent slip bands (PSB) by accumulated local plastic cyclic deformation. The formation of slip bands lead to a local stress increase, which can lead to crack initiation and crack propagation as a function of the local barrier strength. In particular, microstructural features act as effective barriers against slip band transmission and fatigue crack growth, Zhai 2005. The type II behavior occurs predominantly in high-strength steels, it describes the crack initiation and crack growth from non-metallic inclusions at the material surface as well as in the volume, Mughrabi 2002. The critical inclusion size decreases with a decreasing stress amplitude, at the same time a fine granular area (FGA) which is surrounding the inclusions is formed. Within the FGA, due to the accumulation of dislocations, a grain refinement occurs. The grain refinement results in a local reduction of the cyclic stress intensity threshold for fatigue crack initiation, Sakai 2009, Grad et al. 2012. 2. Experimental Procedure The investigated material is a low-alloyed carbon steel 50CrMo4 (German designation: 1.7228). The chemical composition of the material is given in Table 1. The material is austenitized at a temperature of 860 °C and then quenched in oil. The hardness of the material has been consciously changed by a change in parameters for the tempering treatment. The tempering temperature of 550°C leads to a moderate hardness of 37HRC and a yield strength of σ YS =992MPa, and the tempering temperature of 200°C to a hardness of 57 HRC and a yield strength of σ YS =1561MPa, respectively. In this way, the influence of different degrees of hardness and strength on the fatigue mechanisms is investigated.

Table 1: Chemical composition (wt. %), heat treatment parameters and mechanical properties for the steel 50CrMo4.

σ UTS MPa 1095 2128

HRC

Material 50CrMo4

C

Cr

Mo

Mn

P

S

Fe

σ YS

0.48

1.00

0.18

0.71

0.013

0.010

bal.

MPa

37HRC 57HRC

austenitizing: 850°C (0.5h) oil quench; temper heat treatment: 550°C (1.5h) air-cool austenitizing: 850°C (0.5h) oil quench; temper heat treatment: 200°C (1.5h) air-cool

992

37 57

1561

The martensitic microstructure of the material has been investigated with the aid of automated electron backscattering diffraction (EBSD). Although the material has been hot-rolled, there is no evidence of a preferential