The influence of a duplex microstructure in steels on fatigue crack growth in the near-threshold region
- PDF / 2,512,887 Bytes
- 7 Pages / 594 x 774 pts Page_size
- 27 Downloads / 199 Views
I.
INTRODUCTION
IN recent years at least two distinct types of duplex microstructure have been developed in steels. The first of these, and the one of current technical interest because of a good combination of strength and ductility, is obtained by allowing partial transformation of austenite to occur prior to quenching from the a - y region. 1 The resultant microstructure consists of a continuous ferritic network which encapsulated volumes of martensite formed from the residual austenite on quenching. We refer to this microstructure as Type A. The second type of duplex microstructure is obtained by reheating a normalized steel into the a - y region, partially transforming the microstructure to austenite and then quenching. This heat treatment results in a microstructure wherein reverted austenite has transformed to martensite and forms a continuous network encapsulating the ferrite. We refer to this microstructure as Type B, and it is with this microstructure that we are primarily concerned in this paper. The reason for this interest is that in prior work it has been shown that this Type B microstructure can lead to a high threshold, i.e., of the order of 14 MPaX/-~ and yield strength of 430 MPa. 2 In contrast, the corresponding values for a normalized microstructure are 8 MPaX/m and 250 MPa. The purpose of the present study was to further elucidate the cause of the high threshold level in the Type B microstructure through consideration of microcracking processes and crack closure.
II.
MATERIALS AND EXPERIMENTAL PROCEDURE
AISI types 1018, 1045, and 10B35 boron, as well as 2.25 Cr-1 Mo-0.1C steels were used in this study. Nominal chemical compositions for these steels are given in Table I. In the 10B35 two different duplex microstructures (Type B) which consist of different volume fractions of ferrite were produced. In each of the other steels one such microstructure was developed. In addition, normalized microstructures were also developed in the AISI 1018 and 10B35 steels for K. MINAKAWA is Research Associate, Department of Metallurgy and Institute of Materials Science, University of Connecticut, Storrs, CT 06268; Y. MATSUO, formerly Graduate Student, University of Connecticut, is now with Komatsu, Ltd., Komatsu, Japan; and A. J. McEVILY is Professor, Department of Metallurgy and Institute of Materials Science, University of Connecticut, Storrs, CT 06268. Manuscript submitted April 20, 1981. METALLURGICAL TRANSACTIONS A
9
purposes of comparison. The heat treatment schedules used are presented in Table II, and examples of the resultant microstructures are shown in Figure 1. The microstructural characteristics for the duplex microstructures are tabulated in Table III, and the mechanical properties of all materials tested are given in Table IV. The degree of connectivity of the martensite in the duplex microstructure is defined by the parameter $, where $ = N g / N g + N b , and Ng is the average number of intersections with boundaries of the martensitic structure per unit of length, m m -1, and Nb is the av
Data Loading...
