The effect of drawing strain on the fatigue behaviour of stainless and carbon steel wires

Doctoral Thesis


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University of Cape Town

A study has been made of the fatigue crack initiation and fatigue crack growth behaviour of three different steels in wire form, namely, an austenitic AISI 304 stainless steel, a corrosion resistant ferritic steel, 3CR12, and pearlitic high carbon steel. The stainless steel wires were produced in the laboratory at a drawing speed of 50 mm min-1, without intermediate annealing, whilst the high carbon pearlitic steel was manufactured commercially. Studies were made on stainless steel wires as a function of drawing strain between 0.09 and 0.585. Fatigue testing was carried out on an ESH servo hydraulic testing machine on both notched and unnotched samples and the S-N curves were used to evaluate the fatigue properties of the steels. Tests were performed with sinusoidal loading and load ratios of R= 0.048 and R=0.22 at a frequency of 2Hz. The microstructural evolution during drawing was characterised by optical and transmission optical microscopy, and x-ray diffraction. Fatigue crack growth and fracture surfaces were studied using scanning electron microscopy. In general, the fatigue limit was enhanced by increased drawing strain, but such strain also increased the subsequent crack propagation rates. The highest value of fatigue limit of 630 MPa was exhibited by the commercial pearlitic steel despite of its high notch sensitivity. Both shot peening of the steel wire surface and reducing the surface roughness by manual polishing increased the fatigue limit between 40 and 25 % respectively. The fatigue limit of AISI 304 stainless steel wire was improved from 215 MPa to 650 MPa after drawing to 0.585 strain. This improvement is attributed to the deformation-induced phase transformation of (ϒ) austenite to α'-martensite. X-ray diffractometer traces show that the amount of strain-induced martensite varied from 8% in the wires drawn at low strain (0.09) to 36% in the wire samples drawn to 0.585 strain. This study has established that approximately 20% of deformation-induced martensite, through drawing strain, is a critical amount which determines the subsequent fatigue response of this steel. If the amount of previously developed martensite is less than the critical amount of 20%, the martensite formed during the fatigue process will act beneficially by retarding fatigue cracking, raising the fatigue limit and resulting in a ductile fatigue fracture surface. However, in the presence of more than 20% of martensite, any martensite induced by cyclic strain will encourage more rapid crack initiation compared to a material containing less than 20% martensite which leads to more brittle fracture surface characteristics. The fatigue limit of 3CR12 steel wire was also improved from 130 MPa to 310 MPa (maximum stress) after drawing to 0.68 strain. The experimental results indicate that the use of drawn 3CR12 ferritic steel for wire application under cyclic conditions is restricted to low stress levels. However, the application of heat treatment and the resultant development of a dual-phase microstructure, improved the fatigue limit to 470 MPa. Based on the findings in this study, recommendations regarding material selection and drawing process optimisation for wire production to improve the fatigue performance of AISI 304 stainless steel is given.

Includes bibliographical references.