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This paper reviews the bending fatigue endurance limits reported in the literature for carburized steels. Almost an order of magnitude difference in fatigue limits (210-1950 MPa) is reported. The reasons for this wide range are not clear because of the complexity of interactions between the microstructural, testing and manufacturing systems associated with carburized components. In well characterized specimens, two types of microstructures and fatigue crack mechanisms associated with moderate and high levels of fatigue performance have been identified and are described here. Finally, the effects of selected engineering parameters on the measurement of bending fatigue limits are discussed. The need for more complete experimental characterization of fatigue performance is stressed as a critical step to produce consistent bending fatigue performance at the upper end of that attainable in carburized steels.

High strain rate test methods to obtain strain-rate dependent sheet steel tensile properties are considered. A tensile test method for sheet steels was developed to obtain accurate stress-strain data over the strain rate range from 0.001 s-1 to 500 s-1 using a servo-hydraulic test machine and tensile samples instrumented with strain gages. Results on several different automotive sheet steels, including interstitial free (IF), high strength low alloy (HSLA), dual phase (DP), and transformation induced plasticity (TRIP) steels, are presented. The results show that strain rate response differs between the various alloy systems. These results are compared with previously published data on strain-rate dependent steel properties. The importance of stress-strain curve shapes, which depend on alloy system, on energy absorption calculations using areas under stress-strain curves are also described.

Third generation advanced high strength steels (AHSS) have been developed combining high strength and formability, allowing for lightweighting of vehicle structural components. These AHSS components are exposed to paint baking operations ranging in time and temperature to cure the applied paint. The paint baking treatment, combined with straining induced from part forming, may lead to increased in-service component performance due to a strengthening mechanism known as bake hardening. This study aims to quantify the bake hardening behavior of select AHSS grades. Materials investigated were press hardenable steels (PHS) 1500 and 2000; transformation induced plasticity (TRIP) aided bainitic ferrite (TBF) 1000 and 1200; and dual phase (DP) 1000. The number designations of these grades refer to minimum as-received ultimate tensile strengths in MPa. Paint baking was simulated using industrially relevant times and temperatures from 15 to 60 min and 120 to 200 °C, respectively.

Crack initiation and propagation in an SAE 4320 steel gas carburized to a 1.0 mm case depth was examined in specimens subjected to bending fatigue. Cellulose acetate replicas of incrementally loaded specimens showed that small, intergranular cracks were initiated during static loading to stress levels just above the endurance limit. The intergranular cracks arrest and serve as initiation sites for semi-elliptical, transgranular fatigue crack propagation. The maximum depth of stable crack propagation was between 0.17 and 0.23 mm, a depth which corresponds to the maximum hardness of the carburized case. Three equations which provide approximations to the stress distribution in the fatigue specimens were used to calculate KIC for the carburized case with values of maximum applied stress and measured stable crack geometry.

The bending fatigue performance of four heats of carburized, commercially-produced SAE 4320 steel was evaluated. Simulated gear tooth in bending (SGTB) cantilever beam specimens from each heat were identically carburized and fatigue tested in the direct quenched condition after carburizing. The microstructure and fracture surfaces of all specimens were characterized with light and electron microscopy. The four direct quenched sets of specimens performed similarly in low cycle fatigue. Endurance limits among the direct quenched specimens ranged between 1100 and 1170 MPa (160 and 170 ksi) and intergranular cracking dominated fatigue crack initiation. An additional set of specimens from one of the heats was reheated after carburizing. The fatigue performance of the reheated specimens was superior to that of the direct quenched specimens in both the low and high cycle regions. The effects of inclusion content, microstructure, and residual stresses on fatigue performance are discussed.

This study evaluated the bending fatigue performance of a modified SAE 4320 steel as a function of carburizing technique. S-N curves and endurance limits were established by fatigue testing modified Brugger-type specimens that are designed to simulate a single gear tooth. Fractured specimens were examined by light and electron microscopy to determine crack initiation sites, establish the extent of stable crack propagation, and analyze surface oxide types and distributions. Test results show that plasma-carburizing boosted the endurance limit of an oxidation-susceptible gear steel from 1100 MPa to 1375 MPa. Fatigue endurance limits in excess of 1400 MPa had previously been achieved in gas-carburized SAE 4320 steels by reheat treatments and reductions in high-oxidation potential elements. The level of improvement observed in this study suggests that any of these advanced processing techniques can allow significant size reductions and weight savings in automotive transmission gears.

The effects of prestrain and zinc coating type on the bending fatigue behavior of titanium-stabilized interstitial free steel were evaluated. From a single zinc bath chemistry, coated sheet steel samples were prepared with either a hot dip galvanized or galvannealed coating. Uniaxial tensile prestrains of 2 and 4 pct. were introduced parallel to the rolling direction on 12.7 cm wide strips. Krouse-type fatigue samples were machined both parallel and transverse to the rolling/prestrain direction. Reversed bending S-N fatigue data showed that the fatigue resistance depended on a complex interaction between the strength increase due to work hardening and fatigue crack development as altered by the presence of the coatings. For both coating types the fatigue resistance increased with prestrain. During prestrain, coating cracks oriented perpendicular to the tensile prestrain direction developed and the crack density was greater in the galvannealed materials.

A large set of bending fatigue data on carburized steels has been statistically analyzed to quantitatively describe the effects of process and microstructural variables. Increasing demands on gear steels require a broad examination of past bending fatigue research to reveal the primary factors that determine fatigue performance and guide future gear steel design. Fatigue performance was correlated to specimen characteristics such as retained austenite content, case and core grain size, extent of intergranular oxidation, surface roughness, and the case profiles of residual stress, hardness, and carbon content. Prior austenite grain size in the case and surface residual stress were found to most strongly influence bending fatigue endurance limit. A multiple regression model to predict endurance limit achieved an R-squared value of 0.56.

Five heats of vanadium-microalloyed steel with carbon contents from 0.29% to 0.40% and sulfur contents from 0.031% to 0.110% were forged into automotive spindles and air cooled. Three of the steels were continuously cast whereas the other two were ingot cast. The forged spindles were subjected to microstructural analysis, mechanical property testing, full component testing and machinability testing. The microstructures of the five steels consisted of pearlite and ferrite which nucleated on prior austenite grain boundaries and predominantly on intragranularly dispersed sulfide inclusions of the resulfurized grades. Ultimate tensile strengths and room temperature Charpy V-notch impact toughness values were relatively insensitive to processing and compositional variations. The room temperature tensile and room-temperature impact properties ranged from 820 MPa to 1000 MPa (120 to 145 ksi) and from 13 Joules to 19 Joules (10 to 14 ft-lbs), respectively, for the various steels.

Quenching & Partitioning (Q&P) is receiving increased attention as a novel Advanced High Strength Steel (AHSS) processing route as promising tensile properties of the “third generation” have been reported. The current contribution reports hole expansion ratios (HER) of Q&P steels and compares the values with HERs obtained for “conventional” AHSS processing routes such as austempering and Quench & Tempering (Q&T). Intercritically annealed C-Mn-Al-Si-P and fully austenitized C-Mn-Si microstructures were studied. Optimum combinations of tensile strength and HER were obtained for fully austenitized C-Mn-Si Q&P samples. Higher HER values were obtained for Q&P than for Q&T steels for similar tempering/partitioning temperatures. Austempering following intercritical annealing results in higher HER than Q&P at similar tensile strength levels. In contrast, Q&P following full austenitization results in higher hole expansion than austempering even at higher strength levels.

The bending fatigue performance of gears, cantilever beam specimens, and notched-axial specimens were evaluated and compared. Specimens were machined from a modified SAE-4118 steel, gas-carburized, direct-quenched and tempered. Bending fatigue specimens were characterized by light metallography to determine microstructure and prior austenite grain size, x-ray analysis for residual stress and retained austenite measurements, and scanning electron microscopy to evaluate fatigue crack initiation, propagation and overload. The case and core microstructures, prior austenite grain sizes and case hardness profiles from the various types of specimens were similar. Endurance limits were determined to be about 950 MPa for both the cantilever beam and notched-axial fatigue specimens, and 1310 MPa for the single gear tooth specimens.

The effects of deep rolling were evaluated by reviewing the fatigue performance of three medium-carbon (0.4 C) bar steels representing microstructural classes characteristic of forging steels used for crankshaft and other automotive applications. Deep rolling is a surface deformation process whereby a radially symmetric work piece undergoes a surface deformation operation. The steel grades included a quenched and tempered alloy steel (4140) that demonstrated a high yield stress and low strain hardening rate, a non-traditional bainitic experimental grade (1.2 Mn, 0.72 Si) containing high amounts of retained austenite with low yield stress and high strain hardening rate, and a ferritic/pearlitic grade (1.3 Mn, 0.56 Si) with a low yield stress and medium strain rate hardening rate. A reproducible test methodology to assess fatigue behavior was developed, based on flex-beam, fully reversed, S-N type laboratory fatigue testing.

Three heats of 0.40% carbon microalloyed steel, containing either 0.03 % or 0.10% sulfur, and with and without a 0.09% vanadium addition, were subjected to metallographic analysis and mechanical property testing. Bars were heated to austenitizing temperatures, between 1000°C and 1300°C. Significant amounts of intragranular ferrite, which has been associated with improved toughness, formed only in specimens containing vanadium and high sulfur which were austenitized above 1100°C. The balance of the microstructure consisted of ferrite which formed at prior austenite grain boundaries and large amounts of pearlite. High densities of manganese sulfide particles in the steels with high sulfur content effectively retarded austenite grain growth. The formation of significant amounts of intragranular ferrite decreased mean free ferrite spacing, effectively refined the pearlite structure, and lowered the Charpy V-notch impact transition temperature.

This project investigated the effect of carburizing carbon-potential and thermal history on the bending fatigue performance of carburized SAE 4320 gear steel. Modified-Brugger cantilever bending fatigue specimens were carburized at carbon potentials of 0.60, 0.85, 1.05, and 1.25 wt. pct. carbon, and were either quenched and tempered or quenched, tempered, reheated, quenched, and tempered. The reheat treatment was designed to lower the solute carbon content in the case through the formation of transition carbides and refine the prior austenite grain size. Specimens were fatigue tested in a tension/tension cycle with a minimum to maximum stress ratio of 0.1. The bending fatigue results were correlated with case and core microstructures, hardness profiles, residual stress profiles, retained austenite profiles, and component distortion.

Knowledge of high strain-rate deformation behavior of automotive body structural materials is of importance for design of new vehicles with improved crash-energy management characteristics. Since a large range of plastic strains is encountered during the forming process prior to assembly, the mechanical behavior of sheet steels under high strain rate deformation conditions must be understood after pre-straining, in addition to the as-produced condition. This paper presents the compression testing methodology employed to examine these properties, and focuses on the effects of quasi-static pre-strains (from 0 to 20%) on the subsequent behavior of a low carbon interstitial free steel tested over a broad range of strain rates (from 10−2 to 103s−1). The results suggest that the increase in yield stress associated with increasing strain rate is not substantially influenced by prior cold work.

The effect of B and Si additions on fracture and fatigue performance of vacuum carburized 4320 steel and modifications of 4320 steel containing additions of Si (1.0 and 2.0 wt pct) and B (0 and 17 ppm) was evaluated by bending fatigue testing. Three rates of gas quenching, in 10 bar nitrogen and 15 and 20 bar helium, were used to cool specimens after carburizing. The B, protected by Ti additions, together with the Si additions, increased core hardenability. The B/Si modified steels showed no improvement in fatigue resistance, as measured by endurance limits established by 10 million cycle runouts without fracture. However, scanning electron microscopy showed that Si reduced sensitivity to intergranular fracture or quench embrittlement, a major cause of bending fatigue crack initiation, and contributed to variable fatigue performance, with both low-cycle failures and runout performance at applied stresses significantly above measured endurance limits.

The effects of strain path on formability and microstructural evolution with strain in two low-carbon steels were examined. The steels include a 0.008 wt. pct. C batch annealed 0.81 mm thick sheet and a 0.031 wt. pct. C continuously annealed0.74 mm sheet with essentially equivalent mechanical properties (YS: 230 MPa; UTS: 350 MPa; n: 0.18). The steels were subjected to various increments of prestrain in either uniaxial or biaxial tension, and forming limits were assessed in the samples after a strain path change to biaxial or uniaxial tension, respectively. Biaxial stretching prestrain lowers the uniaxial tension forming limit, while uniaxial tensile prestrain raises the biaxial stretching forming limit. The differences in forming response were also correlated with distinct dislocation cell structures. The effects of strain path on formability were shown to correlate with predictions based on a redundant strain model and a critical thickness strain model.

The effects of subzero treatments on the bending fatigue performance of carburized gear steels were investigated by cantilever bending fatigue testing. Specimens were machined from SAE-4320 and SAE-9310 bar stock steel, gas-carburized, quenched, tempered at 175°C, subzero cooled to -73°C and -196°C, and tempered at 175°C. Bending fatigue specimens were characterized by light metallography to determine microstructure and prior austenite grain size, x-ray diffraction for residual stress and retained austenite contents, microhardness testing, and scanning electron microscopy to evaluate fatigue crack initiation, propagation and overload. Refrigeration treatments caused additional transformation of retained austenite and increased surface hardness and compressive residual stresses. Bending fatigue endurance limits for the SAE-4320 specimens were determined to be 1310 MPa for the as-carburized condition, 1170 MPa for the -73°C condition, and 1280 MPa for the -196°C condition.

The effects of elevated testing temperature on the fatigue behavior of carburized steel were evaluated by testing modified Brügger bending fatigue specimens at room temperature, 90 °C and 150 °C. SAE 4023, SAE 4320, and SAE 9310 steel were studied to assess the influence of alloy content and stability of retained austenite. Fatigue samples were gas-carburized and tested in air at 30 Hz with a stress ratio of 0.1. An infrared spot lamp was used to heat samples to 90 °C (150 °F) or 150 °C (302 °F) during testing. S-N curves were developed for the room temperature baseline tests as well as elevated temperature tests. The endurance limits determined are as follows: SAE 4023-RT (1170 MPa), SAE 4023-90°C (1140 MPa), SAE 4320-RT (1210 MPa), SAE 4320-90°C (1280 MPa), SAE 9310-RT (1380 MPa), SAE 9310-90°C (1240 MPa).

The effects of several variables on pitting fatigue life of carburized steels were analyzed using a geared roller test machine (GRTM). The material variables that were primarily used to influence retained austenite include aim surface carbon concentration (0.8 % and 0.95 %), alloy (SAE 4320 and a modified SAE 4122), and cold treatment (performed on one material condition per alloy). Testing variables included contact stress in addition to a variation in lambda ratio (oil film thickness/surface roughness), arising from variation in roughness among the machined surfaces. Test results are presented, and differences in performance are considered in terms of material and testing variables. A primary observation from these results is an improvement in contact fatigue resistance apparently arising from cold-treatment and the associated reduction of retained austenite at the surface.