The notch-fatigue limit and notch sensitivity of 0.1-0.6%C-1.5%Si-1.5%Mn transformation-induced plasticity (TRIP)-aided martensitic steels (TM steels) were investigated for use as common rails in nextgeneration automotive diesel engines. Also, these properties were related to the microstructural and retained austenite characteristics. When TM steels containing 0.2% to 0.4% C were subjected to heat treatment for isothermal transformation at 50°C and subsequent partitioning at 250°C, the steels achieved much higher notch-fatigue limits and lower notch sensitivities than those of conventional 0.2-0.4%C-1.0%Cr-0.2%Mo structural steels. This was principally associated with (i) plastic relaxation of localized stress concentration as a result of strain-induced transformation of 3-5 vol% metastable retained austenite and (ii) a large amount of finely dispersed martensite-austenite phase along prior austenitic, packet and block boundaries, as well as (iii) a small amount of carbide only in the wide lath-martensite structure, which may contribute to making fatigue crack initiation and/or propagation difficult.KEY WORDS: notch-fatigue strength; notch sensitivity; ultrahigh-strength steel; TRIP-aided steel; martensite; retained austenite; M-A constituent.
Cr, Mo and/or Ni were added to TRIP-aided bainitic ferrite (TBF) steel (0.2% C, 1.5% Si, 1.5% Mn and 0.05% Nb ultrahigh-strength TBF steel) in order to increase its hardenability. In addition, the effects of the alloying elements on the Vickers hardness, microstructure and retained austenite characteristics of the TBF steels were investigated. When the TBF steels were austempered at temperatures between MS and Mf, the Vickers hardness increased from HV300 to HV430 with increasing hardenability. The microstructure consisted of martensite and bainitic ferrite lath structures and retained austenite phases and the volume fraction of retained austenite increased with increasing hardenability. Conversely, the carbon concentration of the retained austenite decreased with increasing hardenability. Simultaneously, the quantity of the hard blocky martensite phase (M-A constituent) with refined interlath retained austenite films increased with increasing hardenability. These characteristics are mainly caused by the delayed bainite transformation during austempering through the addition of Cr, Mo and/or Ni. The addition of Ni lowered the T0 line further. The retained austenite phases of Cr-and/or Mo-bearing TBF steels were relatively stable against straining, despite their low carbon concentrations.
Fatigue strength of newly developed TRIP-aided annealed martensitic steels (TAM steels) with chemical composition of 0.4%C, 1.5%Si, 1.5%Mn, 0-1.0%Cr, 0-0.2%Mo, 0.05%Nb, 0-18ppmB was examined for application of automotive diesel engine common rail. The TAM steels achieved higher fatigue limits than the conventional structural steels. Also, the TAM steels exhibited extremely high notch fatigue limits and low notch-sensitivity, especially in steels with B or without Cr and Mo. This was associated with TRIP effect of a large amount of metastable retained austenite which suppressed the crack initiation and growth due to plastic relaxation and formation of hard martensite resulting from the strain-induced transformation.
To develop a transformation-induced plasticity (TRIP)-aided bainitic ferrite steel (TBF steel) with high hardenability for a common rail of the next generation diesel engine, 0.2%C-1.5%Si-1.5%Mn-0.05%Nb TBF steels with different content of Cr, Mo and Ni were produced. And, notch-fatigue strength of the TBF steels was investigated and was related to the microstructural and retained austenite characteristics. If Cr, Mo and/or Ni were added to the base steel, the steels achieved extremely higher notch-fatigue limits and lower notch-sensitivity than base TBF steel and the conventional structural steels. This was mainly associated with (i) carbide-free and fine bainitic ferrite lath structure matrix without pro-eutectoid ferrite, (ii) a large amount of fine metastable retained austenite and (iii) blocky martensite phase including retained austenite, which may suppress a fatigue crack initiation and propagation.
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