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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.

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.

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.

Emissions of six 32 passenger transit buses were characterized using one of the West Virginia University (WVU) Transportable Heavy Duty Emissions Testing Laboratories, and the fixed base chassis dynamometer at the Colorado Institute for Fuels and High Altitude Engine Research (CIFER). Three of the buses were powered with 1997 ISB 5.9 liter Cummins diesel engines, and three were powered with the 1997 5.9 liter Cummins natural gas (NG) counterpart. The NG engines were LEV certified. Objectives were to contrast the emissions performance of the diesel and NG units, and to compare results from the two laboratories. Both laboratories found that oxides of nitrogen and particulate matter (PM) emissions were substantially lower for the natural gas buses than for the diesel buses. It was observed that by varying the rapidity of pedal movement during accelerations in the Central Business District cycle (CBD), CO and PM emissions from the diesel buses could be varied by a factor of three or more.

The effect of diesel cetane number, total aromatic content T90, and fuel nitrogen content on regulated emissions (HC, CO, NOx, and PM) from a 1991 DDC Series 60 engine were measured Emissions tests were conducted using the EPA heavy-duty transient test (CFR 40 Part 86 Subpart N) at a laboratory located 5,280 feet (1609 m) above sea level. The objective of this work was to determine if the effect of fuel chemistry at high altitude is similar to what is observed at sea level and to examine the effect of specific fuel chemistry variables on emissions. An initial tea series was conducted to examine the effect of cetane number and aromatics. Transient emissions for this test series indicated much higher (50 to 75%) particulate emissions at high altitude than observed on the same model engine and similar fuels at sea level.

Natural gas presents several challenges to engine manufacturers for use as a heavy-duty, lean burn engine fuel. This is because natural gas can vary in composition and the variation is large enough to produce significant changes in the stoichiometry of the fuel and its octane number. Similarly, operation at high altitude can present challenges. The most significant effect of altitude is lower barometric pressure, typically 630 mm Hg at 1600 m compared to a sea level value of 760 mm. This can lower turbocharger boost at low speeds leading to mixtures richer than desired. The purpose of this test program was to determine the effect of natural gas composition and altitude on regulated emissions and performance of a Cummins B5.9G engine. The engine is a lean-burn, closed loop control, spark ignited, dedicated natural gas engine. For fuel composition testing the engine was operating at approximately 1600 m (5,280 ft) above sea level.

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.

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.

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 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.

Punch-stretch tests to determine formability of type 304 stainless steel sheet were conducted using a hemispherical dome test. Sheets of 19.1 mm width and 177.8 mm width were stretched on a 101.6 mm diameter punch at punch rates between 0.042 to 2.12 mm/sec with three lubricant systems: a mineral seal oil, thin polytetrafluoroethelyne sheet with mineral seal oil, and silicone rubber with mineral seal oil. The resulting strain distributions were measured and the amount of martensite was determined by magnetic means. Increasing lubricity resulted in more uniform strain distributions while increased punch rates tended to decrease both strain and transformation distributions. High forming limit values were related to the formation of high and uniformly distributed martensite volume fractions during deformation. The results of this study are interpreted with an analysis of the effects of strain and temperature on strain induced martensite formation in metastable austenitic stainless steels.

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 objective of the work described here is to test the performance of closed-loop controlled, heavy-duty CNG engines in-use, on fuels of different methane content; and to compare their performance with similar diesel vehicles. Performance is measured in terms of pollutant emissions, fuel economy, and driveability. To achieve this objective, three buses powered by closed-loop controlled, dedicated natural gas engines were tested on the heavy-duty chassis dynamometer facility at the Colorado Institute for Fuels and High Altitude Engine Research (CIFER). Emissions of regulated pollutants (CO, NOx, PM, and THC or NMHC), as well as emissions of alde-hydes for some vehicles, are reported. Two fuels were employed: a high methane fuel (90%) and a low methane fuel (85%). It was found that the NOx, CO, and PM emissions for a given cycle and vehicle are essentially constant for different methane content fuels.

The influence of coatings on fatigue behavior has been examined for the following commercially produced sheet steels: uncoated titanium stabilized interstitial-free (IF); electrogalvanized titanium stabilized IF; hot-dip galvanized aluminum killed, drawing quality (AKDQ); and galvannealed AKDQ. Fully reversed bending fatigue tests were conducted at ambient temperature on Krouse-type flexural fatigue machines. A dependence of crack development was observed and correlated to the microstructure and properties of the different coatings. Furthermore, a functional design relationship for each material was determined through stress-life analysis. The experimentally determined fatigue properties were compared to conventional estimates based on tensile properties which ignore coating effects. The results of this work suggest that ductile coatings may enhance fatigue resistance, while brittle coatings may reduce fatigue life.

A revolutionary interplanetary rapid transit concept for transporting scientists and explorers between Earth and Mars is presented by Global Aerospace Corporation under funding from the NASA Institute for Advanced Concepts (NIAC) with support from the Colorado School of Mines, and Science Applications International Corporation. We describe an architecture that uses highly autonomous spaceships, dubbed Astrotels; small Taxis for trips between Astrotels and planetary Spaceports; Shuttles that transport crews to and from orbital space stations and planetary surfaces; and low-thrust cargo freighters. In addition we discuss the production of rocket fuels using extraterrestrial materials; aerocapture to slow Taxis at the planets; and finally describe a number of trade studies and their life-cycle cost results.

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.