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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 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 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 fatigue behavior of five microalloyed steels, processed with hardnesses between 25-28 HRC containing microstructures ranging from precipitation-hardened ferrite-pearlite to bainite, were evaluated in both low cycle (strain controlled) and high cycle (stress controlled fatigue. The vanadium-bearing steels included, 15R30V, 1522 MoVTi, 1522 MoVTiS, 1534 MoVTi, and 1534 MoVTiSi. Conventional quench and tempered 4140 steel was used as a reference. Low cycle fatigue (LCF) data for all steels were similar. Subtle microstructural-dependent differences in the high-strain amplitude region of the LCF curves were attributed to the effects of retained austenite, present in some of the non-traditional bainitic steels. In high cycle fatigue, all steels exhibited similar properties, except for the ferrite-pearlite steel (15R30V) which exhibited the lowest endurance limit, an observation which was attributed to crack nucleation in coarse-grained ferrite.

A series of bending fatigue tests were conducted and S-N data were obtained for two groups of 4320 steel samples: (1) carburized, quenched and tempered, (2) carburized, quenched, tempered and shot peened. Shot peening improved the fatigue life and endurance limit. The S-N data exhibited large scatter, especially for carburized samples and at the high cycle life regime. Sample characterization work was performed and scatter bands were established for residual stress distributions, in addition to fracture and fatigue properties for 4320 steel. Moreover, a fatigue life analysis was performed using fracture mechanics and strain life fatigue theories. Scatter in S-N curves was established computationally by using the lower bound and upper bound in materials properties, residual stress and IGO depth in the input data. The results for fatigue life analysis, using either computational fracture mechanics or strain life theory, agreed reasonably well with the test data.

This book is a comprehensive guide to the broad, dynamic physical metallurgy of steels. Contents include the: interrelationships between chemistry, processing, structure, and performance--the elements of physical metallurgy--are integrated for all the types of steel; evolution, characterization, and performance of steel microstructures are described, with increased emphasis on deformation and fracture; thermal processing and its effect on steels; low carbon sheet steels, together with strengthening mechanisms, developments in microalloyed forging steels, steels with bainitic and a variety of ferritic microstructures, quench and tempered steel performance, high-carbon steels for rail and ultra-high strength wire, and the causes of low toughness and embrittlement Principles, concepts, and understanding of microstructural evolution and performance, within the framework of processing and properties, are illustrated, by plots of data, micrographs and schematic diagrams.

In the second edition of this best-selling book, new information and references are integrated into chapters. Emphasis is still on processing, alloying, microstructure, deformation, fracture and properties of major steel types ranging from low-carbon sheet steels, pearlitic rail and wire steels, to quench and tempered medium- and high-carbon martensitic steels. Microstructural aspects of steelmaking, hardenability, tempering, surface hardening, and embrittlement phenomena are updated, and chapters on stainless and tool steels remain in the second edition. The work is intended to be tutorial and is an essential state-of-the-art reference for anyone that makes, uses, studies and designs with steel.

Forging simulations with a 1522 steel microalloyed by additions of 0.25% Mo, 0.13% V and 0.01% Ti were performed on a laboratory thermomechanical processing simulator. The forging conditions included a strain rate of 22s-1, 50% strain, and temperatures in the range from 1200°C to 950°C. The true stress was found to increase with decreasing deformation temperature for all values of instantaneous true strain. The maximum flow stress increased two-fold as deformation temperature decreased from 1200°C to 950°C, and the recrystallized austenite grain size decreased by a factor of two for this same decrease in temperature. Microstructures evolve from bainitic/ferritic at a cooling rate of 1.4°C/s, to fully martensitic at 16.8°C/s, independent of deformation temperature. Room temperature hardnesses depended primarily on cooling rate and were essentially independent of deformation temperature.

The effects of austenite grain size on the bending fatigue crack initiation and fatigue performance of gas carburized, modified 4320 steels were studied. The steels were identical in composition except for phosphorus concentration which ranged between 0.005 and 0.031 wt%. Following the carburizing cycle, specimens were subjected to single and triple reheat treatments of 820°C for 30 minutes to refine the austenite grain structure, and oil quenched and tempered at 150°C. Specimens subjected to bending fatigue were characterized by light metallography to determine microstructure and grain size, X-ray analysis for retained austenite and residual stress measurements, and scanning electron microscopy for examination of fatigue crack initiation and propagation. The surface austenite grain size ranged from 15 μm in the as-carburized condition to 6 and 4 μm diameter grain size for the single and triple reheat conditions, respectively.