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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.

This work establishes the relationship between core hardness, case hardness, and case depth on susceptibility to hydrogen embrittlement of case hardened steel fasteners. Such fasteners have a high surface hardness in order to create their own threads in a mating hole, and are commonly used to attach bracketry and sheet metal in automotive applications. While case hardened fasteners have been studied previously, there are currently no processing guidelines supported by quantitative data for fastener standards. Through sustained load embrittlement testing techniques, the susceptibility of case hardened steel tapping screws to internal and environmental hydrogen embrittlement is examined. Further characterization of the fastener samples through microhardness testing, microstructure review, and fracture surface examination allows the investigation of susceptibility thresholds. It is shown that core hardness is the primary consideration for susceptibility.

The strain-induced diffusionless shear transformation of retained austenite to martensite during straining of transformation induced plasticity (TRIP) assisted steels increases strain hardening and delays necking and fracture leading to exceptional ductility and strength, which are attractive for automotive applications. A novel technique that provides the retained austenite volume fraction variation with strain with improved precision is presented. Digital images of the gauge section of tensile specimens were first recorded up to selected plastic strains with a stereo digital image correlation (DIC) system. The austenite volume fraction was measured by synchrotron X-ray diffraction from small squares cut from the gage section. Strain fields in the squares were then computed by localizing the strain measurement to the corresponding region of a given square during DIC post-processing of the images recorded during tensile testing.

Fracture split forged steel connecting rods are utilized in many new high performance automotive engines to increase durability. Higher strength levels are needed as the power density increases. Fracture splitting without plastic deformation is necessary for manufacturability. Metallurgical design is a key for achieving the required performance levels. Several medium carbon steels containing 0.07 wt pct P, 0.06 wt pct S and various amounts of Mn, Si, V, and N were produced by vacuum induction melting laboratory heats and hot working the cast ingots into plates. The plates were cooled at varying rates to simulate typical cooling methods after forging. Microstructures were generally ferrite and pearlite as evaluated by light optical and scanning electron microscopy. Mechanical properties were determined by standard tensile tests, high strain rate notched tensile tests, and Charpy V-notch impact tests to assess “splittability”.

Quenching and partitioning (Q&P) is a novel heat treatment to produce third generation advanced high-strength steels (AHSS). The influence of carbon on mechanical properties of Q&P treated CMnSi-steels was studied using 0.3C-1.5Mn-1.5Si and 0.4C-1.5Mn-1.5Si alloys. Full austenitization followed by two-step Q&P treatments were conducted using varying partitioning times and a fixed partitioning temperature of 400 °C. The results were compared to literature data for 0.2C-1.6Mn-1.6Si, 0.2-3Mn-1.6Si and 0.3-3Mn-1.6Si Q&P treated steels. The comparison showed that increasing the carbon content from 0.2 to 0.4 wt pct increased the ultimate tensile strength by 140 MPa per 0.1 wt pct C up to 1611 MPa without significantly decreasing ductility for the partitioning conditions used. Increased alloy carbon content did not substantially increase the retained austenite fractions. The best combinations of ultimate tensile strength and total elongation were obtained using short partitioning times.

In this paper, a two-dimensional microstructure-based finite element modeling method is adopted to investigate the effects of material parameters of the constituent phases on the macroscopic tensile behavior of Q&P steel and to perform a computational material design approach for performance improvement. For this purpose, a model Q&P steel is first produced and various experiments are then performed to characterize the model steel. Actual microstructure-based model is generated based on the information from EBSD, SEM and nano-indentation test, and the material properties for the constituent phases in the model are determined based on the initial constituent properties from HEXRD test and the subsequent calibration of model predictions to tensile test results. The influence of various material parameters of the constituents on the macroscopic behavior is then investigated.

In this study, a procedure for characterizing advanced high strength steel sheets is presented in view of determining the material parameters for constitutive models that can be used for accurate prediction of springback and sidewall curl. The mechanical properties of DP980 and TRIP780 sheets were obtained experimentally, and their cyclic tension-compression behaviour was modeled with the Chaboche nonlinear kinematic hardening model and the Yoshida-Uemori two-surface plasticity model that are implemented in LS-DYNA. The unloading moduli were determined from monotonic tension tests at various prestrain levels. An inverse approach based on linear and quadratic response surfaces created by Sequential Strategy with Domain Reduction (SRSM) methodology using LS-OPT software was used and investigated to identify specific material parameters in each constitutive model.

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.

This paper reports the development of new fuel ignition quality and combustion experiments performed using the Ignition Quality Tester (IQT). Prior SAE papers (961182, 971636, 1999-01-3591, and 2001-01-3527) documented the development of the IQT constant volume combustion chamber experimental apparatus to measure ignition qualities of diesel-type fuels. The ASTM International test method D6890 was developed around the IQT device to allow the rapid determination of derived cetane number (DCN). Interest in chemical kinetic models for the ignition of diesel and biodiesel model compounds is increasing to support the development of advanced engines and fuels. However, rigorous experimental validation of these kinetic models has been limited for a variety of reasons. Shock tubes and rapid compression machines are typically limited to premixed gas-phase studies, for example.

The susceptibility of Advanced High Strength Steels (AHSS) to hydrogen embrittlement (HE) was evaluated on selected high strength sheet steels (DP 600, TRIP 780, TRIP 980, TWIP-Al, TWIP, and Martensitic M220) and the results were compared to data on a lower strength (300 MPa tensile strength) low carbon steel. Tensile samples were cathodically charged and then immediately tensile tested to failure to analyze the mechanical properties of the as-charged steel. The effects of hydrogen on deformation and fracture behavior were evaluated through analysis of tensile properties, necking geometry, and SEM images of fracture surfaces and metallographic samples of deformed tensile specimens. The two fully austenitic TWIP steels were resistant to hydrogen effects in the laboratory charged tensile samples.

Knowledge of the net thinning strain that occurs in a sheet as it is bent over a single radius is an important component in understanding sheet metal formability. The present study extends the initial work of Swift on thinning during plane-strain bending to sheet steels with power law stress-strain behavior and with the inclusion of friction. The experimental data come from studies on the enhanced forming limit curve on DQSK steel and analysis of the curl behavior of 590R and DP600 steels. Results for single radius bending from these studies are used in the present investigation. It has been found that the amount of net thinning strain depends on back tension, initial plane-strain yield strength, and the maximum true bending strain calculated for the neutral plane at the mid-thickness of the sheet.

Drawbeads in production stamping dies often have insufficient penetration of the male bead into the female cavity. With insufficient penetration, the actual bending radii of the sheet metal are larger than the geometrical radii of the drawbead. The actual bending radii in the sheet directly affect the force that restrains sheet movement. To predict the restraining stress due to a drawbead, it is necessary to know the actual bending radii in the sheet as it passes though the drawbead. Data from a previous study are used to develop empirical regression equations for predicting measured radii of the sheet that is bent around the radii in a drawbead. A physical model for the evolution of the sheet radii as the drawbead closes is proposed. This model is consistent with the empirical equations and the mechanics of the sheet bending process.

The bending fatigue performance of high temperature (1050 °C) vacuum carburized Nb modified 8620 steel, with niobium additions of 0.02, 0.06 and 0.1 wt pct, was evaluated utilizing a modified Brugger specimen geometry. Samples were heated at two different rates (20 and 114 °C min-1) to the carburizing temperature resulting in different prior austenite grain structures that depended on the specific Nb addition and heating rate employed. At the lower heating rate, uniform fine grained prior austenite grain structures developed in the 0.06 and 0.1 Nb steels while a duplex grain structure with the presence of large (>200 μm grains) developed in the 0.02 Nb steel. At the higher heating rate the propensity for abnormal grain growth was highest in the 0.02 Nb steel and complete suppression of abnormal grain growth was achieved only with the 0.1 Nb steel.

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.

In stamping advanced high strength steels (AHSS), the deviations from desired part geometry caused by springback from a radius, curl, twist, and bow are major impediments to successfully producing AHSS parts. In general, the conventional elastic modulus is used to quantify the strain that occurs on unloading. This unloading strain causes deviations from desired part geometry. Considerable evidence in the literature indicates that for tensile testing, the conventional elastic modulus does not accurately describe the unloading strain. The present study uses new data and results from the literature to examine the average slope of tensile stress strain curves on unloading. This slope is termed the effective unloading modulus. The results from this study quantitatively describe how the effective unloading modulus decreases with increasing strength, prestrain, and unloading time.

Prediction of fatigue performance of large structures and components is generally done through the use of a fatigue analysis software, FEA stress/strain analysis, load spectra, and materials properties generated from laboratory tests with small specimens. Prior experience and test data has shown that a specimen size effect exists, i.e. the fatigue strength or endurance limit of large members is lower than that of small specimens made of same material. Obviously, the size effect is an important issue in fatigue design of large components. However a precise experimental study of the size effect is very difficult for several reasons. It is difficult to prepare geometrically similar specimens with increased volume which have the same microstructures and residual stress distributions throughout the entire material volume to be tested. Fatigue testing of large samples can also be a problem due to the limitation of load capacity of the test systems available.

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.

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).

Most environments inhabited by living beings have dust. Much of that dust comes from the continuous flaking of our own skin and atmosphere borne particles of submicron size. Dust mites seem to play an important role in integrating fine scale dust, which they consume to grow, resulting in larger length scale dust. Dust continues to agglomerate and grow. Air filters are designed to remove dust and control dust agglomeration. We report a simple scaling law, based on kinematic simulations of dust filtration through a one dimensional idealized filter. The results reveal insights regarding how filters may clog in time. Our results may be of use to someone interested in designing customized air filters for optimum dust removal in an environment with a known dust distribution.

With the increased use of tubular steel products, especially for automotive hydroforming applications, there is increased interest in understanding the mechanical properties measured by tensile tests from specimens of different orientations in the tube. In this study, two orientations of tensile specimens were evaluated -- axial specimens with and without flattening and flattened circumferential specimens. Three steels were evaluated -- two thicknesses of aluminum killed drawing quality (AKDQ) steel and one thickness of high strength low alloy (HSLA) steel. Mechanical property data were obtained from the flat stock, conventional production tubes and quasi tubes. Quasi tubes were produced from the flat stock on a 3-roll bender, but the quasi tube was not welded or sized.