Effects of non-metallic inclusions on fatigue properties of calcium treated steels
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Date
2004-12-10
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en
Pages
102
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Teknillisen korkeakoulun Materiaalitekniikan laboratorion julkaisuja, Teknillisen korkeakoulun Materiaalitekniikan laboratorion julkaisuja, 4/04
Abstract
Fatigue behaviour of 22 industrial test charges of AISI 8620 carburizing steel with two different calcium treatment levels was studied. The research work consisted mainly of rotating bending fatigue tests, residual stress and surface roughness measurements, electron microscopy, different steel cleanliness level and statistical inclusion size estimation methods. There were no significant differences between the σw/Rm ratios of the casts with the large amount of calcium injection and the casts with the small amount of calcium injection. In the casts with the large amount of calcium injection, the fatigue cracks initiated mostly from the surface and interior inclusions. In the casts with the small amount of calcium injection, the fatigue cracks initiated mostly from the surface discontinuities. The inclusions responsible for fatigue crack initiation were in the most cases calcium aluminates encapsulated in calcium sulfide containing small amounts of magnesia and/or silica. The fatigue crack initiation from cracked and non-cracked inclusions resulted in similar fatigue life on the same ΔK level. The fatigue strength scatter was larger in the casts with the large amount of calcium injection. In rotating bending fatigue the σw/Rm ratio was almost independent of inclusion size in the average fatigue crack initiating inclusion size region smaller than ∼ 70-90 µm. The results of DIN 50 602 and SFS-ENV 10247 inclusion rating methods and ultrasonic tests in immersion did not correlate with the inclusions that were responsible for fatigue failure in these steels. The results may, however, suggest guidelines for the fatigue properties and the machinability of these steels when the contents of certain alloying elements are taken into account. Ultrasonic tests in immersion provide more relevant information about the fatigue properties and machinability of these steels than the conventional inclusion rating methods do, but its resolution capability still needs improvement. In most casts the maximum inclusion sizes predicted by the statistics of extreme value method were much smaller than the size of the inclusions found at the fatigue crack initiation sites of the fatigue specimens. The studied steels seemed to have two different inclusion size distributions, i.e., the inclusions detected at the polished microsections and the inclusions at the fatigue crack initiation sites. Both distributions had similar morphology and chemical composition, which was contrary to the earlier findings of the bilinear nature of inclusion distribution in some steels. The successful application of the Murakami-Endo model with these steels requires quite a large inspection area, approximately 8400 mm2 at least, to enable the detection of the population of the largest inclusions, which are responsible for fatigue failure.Description
Keywords
fatigue strength, nonmetallic inclusions, calcium treated steel, statistical analysis, statistics of extremes, ultrasonic testing, machinability