Notch-fatigue Strength of Advanced TRIP-aided Martensitic Steels

Abstract

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.

Notch-Fatigue Properties of Advanced TRIP-Aided Bainitic Ferrite Steels

“…On the contrary, the volume fraction increases with hardenability except for steel E, with a considerable increase in volume fractions of second phase. Kobayashi et al [14] have According to previous reports, [2,3] retained austenite stability against the strain-induced transformation for TRIP-aided steel can be evaluated by "strain-induced transformation factor; k" defined by the following equation, rather than the carbon concentration, if retained austenite morphology considerably changes. [2,3,14] k values between 1 and 5 means to have nearly the same stability and to be stable enough.…”

“…Kobayashi et al [14] have According to previous reports, [2,3] retained austenite stability against the strain-induced transformation for TRIP-aided steel can be evaluated by "strain-induced transformation factor; k" defined by the following equation, rather than the carbon concentration, if retained austenite morphology considerably changes. [2,3,14] k values between 1 and 5 means to have nearly the same stability and to be stable enough. In steel A, relatively low retained austenite stability despite high carbon concentration is caused by isolated morphology, differing from morphology surrounded by narrow martensite in steels B-E. Table II shows Vickers hardness and tensile properties of steels A-H. Vickers hardness of the steels A-E are between HV242 and HV430 and increases with increasing hardenability.…”

Microstructure and Mechanical Properties of a TRIP-Aided Martensitic Steel

“…Fatigue tests were carried out using a multi-type fatigue testing machine at 25°C, with a sinusoidal wave of 80 Hz. The stress ratio, defined as the ratio of minimum stress (r min ) to maximum stress (r max ) was R = 0.1 [18,19]. The fatigue limit was defined by the maximum value of the stress amplitude (r R = r max -r min ) without failure up to 1.0 9 10 7 cycles.…”

“…With this is in mind, the superior impact toughness of TM steel is believed to be caused by the presence of (i) a softened wide lath-martensite matrix, which contains only a small amount of carbide and hence has a lower carbon concentration, (ii) a large quantity of a finely dispersed MAlike phase, and (iii) a metastable retained austenite phase that occupies 2-4 vol% of the MA-like phase, which subsequently leads to plastic relaxation via its strain-induced transformation. Figure 14 shows the fatigue limits of smooth and notched specimens (FL, FL N ) and the notch sensitivity (q) of TM steels [19]. Note that the notch sensitivity is defined by the following equation [32]:…”

“…Because TM steel also possesses superior mechanical properties such as toughness [15][16][17], fatigue strength [18,19], and hydrogen embrittlement resistance [20] as compared to conventional structural steel (Fig. 2), application of TM steel to automotive drivetrain components such as gears, drive shafts, CV joints, clutch plates, etc.…”