Geometric features of MnS grains in damaged free-cutting steel SUM24L by fatigue and dynamic tension

Abstract

For free-cutting steels, their mechanical properties, especially local toughness, are functionally degraded. The second phase exists in steel matrix, as a chip breaker, tool edge stabilizer and tool life extender. For these purposes, the second phase plays the role of a solid lubricant. Typical materials as the second phase in free-cutting steels are Pb and MnS. The second phase grains are recognized as stress concentrated points during machining. Such points have the possibility to be new crack initiation sites during fatigue loading. Formerly, the present author found that some Al alloys are degraded by loading combination of fatigue and dynamic tension. On the other hand, for some structural steels, almost no degradation is found by the fatigue-impact loading in tension. However, in 2013 the present author reported that free-cutting steels are deteriorated to some extent by such a loading combination. The deterioration may be caused by the stress concentrated points around the second phase, MnS in SUM24L (JIS G 4804:2008, equivalent to AISI 12L14 steel), which is weakened by pre-fatigue process. In this paper, in order to detect microscopic difference between the specimens fractured in dynamic tension with/without pre-fatigue, geometric features of MnS grains are observed by an optical microscope. In the microscopic observation, typical deformation of surrounding matrix steel is found. Into MnS grain, steel matrix is penetrated. Focusing on the existence of the matrix penetration, the number of MnS grains in free-cutting steel is counted with geometrical features. The experimental results show that the information of the penetrations has a possibility to be an indicator of the dynamic strain rate, comparing with the specimens fractured by quasi-static tension.

Comparison of penetrated MnS grains by steel matrix in variously loaded free-cutting steels with/without Pb as one of the workability parameters

“…The pre-fatigue loading expected to accelerate the degradation due to the change of interfacial situation between steel matrix and the second phase, mainly MnS grain. Itabashi and Koseki (2013) reported low-cycle pre-fatigue gave 10% reduction of quasistatic strength and 20% reduction of dynamic one, for the same lot steels of this series of experiments. As explained earlier, the maximum (pre-fatigue) stress of 100 MPa was almost the onset of the degradation.…”

“…After tensile fracture, the number of such penetrations drastically increased, because transverse compression was induced during necking formation. Formerly, the present author thought that this phenomenon could be utilized for the indicator of loading history of fractured free-cutting steel (Itabashi, 2015).…”

Effect of pre-fatigue on mechanical properties of modified double-base propellant

“…Depending on the previous work on the subject in question, the effect of prefatigue on stress-strain properties may vary depending on the material type and fatigue loading parameters. For example, alloy hardening was found for 2017A-T3 [4] , followed by material softening, and for AISI 4140T [3] , Softening only occurs in AISI 1215 and AISI 12L14 [6] steels, while the above characteristics are absent in 5544-O [4] and 6061-T6 [3] alloys. In addition, the development of fatigue damage may lead to a significant decrease in fracture strain [5] .…”

“…The microstructure of pre-fatigue specimens is affected by cyclic softening or hardening effect and microscopic damage. Studies conducted in this area have clearly shown that due to previous cyclic loading, the elongation of 6061-T6 aluminum alloy decreased by 9%, AISI 4140T steel decreased by 63% [3] , 2017A-T3 aluminum alloy decreased by 70%, and 5544-O alloy decreased by 6% [4] , The decrease was 60% for Ti-6Al-4V titanium alloy [5] , 4% for AISI 1215 steel and 10% for AISI 12L14 steel [6] . Depending on the previous work on the subject in question, the effect of prefatigue on stress-strain properties may vary depending on the material type and fatigue loading parameters.…”

New Medium-Carbon Free-Machining Steels Containing Bismuth and Calcium

“…Free-machining steels containing sulfur and lead are widely used for production of precision small parts machined by various metalworking equipment, especially automatic turning lathes and barstock machines [1][2][3][4][5][6]. But the tendency for limitation of environmentally harmful substances is growing, and modern regulations restrict the use of heavy metals including lead [7][8][9][10]. Producers of automobiles and office devices carry out intensive research to replace lead-bearing steels presently in use with new lead-free steels having in view the expected further restriction of environmental norms.…”