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Title: Effects of Silicon and Boron Additions on the Susceptibility to Quench Embrittlement and the Bending Fatigue Performance of Vacuum Carburized Modified 4320 Steel

Abstract

The potential to reduce the susceptibility to quench embrittlement, a fracture mechanism that promotes intergranular fracture in high carbon steels, and to improve the bending fatigue performance of vacuum carburized modified SAE 4320 steels was evaluated. Data were obtained on an industrially produced SAE 4320 steel and four laboratory produced steels based on the 4320 composition but with additions of Si (1.0 or 2.0 wt pct) and B (0 or 17 ppm). All five alloys were vacuum carburized together and gas quenched with three different cooling rates as controlled by the gas quench conditions: 10 bar nitrogen, and 15 and 20 bar helium. Modified Brugger fatigue samples of each alloy and quench condition were tested in cantilever bending and failed samples were analyzed with scanning electron and Auger spectroscopy. Standard S-N curves and endurance limits were obtained and the fracture surfaces were evaluated using both light and electron microscopy techniques to determine fracture initiation sites and fracture growth mechanisms, both in the stable fatigue crack growth zone and in the overload zone. The percentage of transgranular fracture in the carburized case was quantified and used as a measure of the susceptibility to quench embrittlement. The susceptibility to quench embrittlement wasmore » observed to be independent of quench rate and boron additions, but depended on Si content. With an increase in Si content, the extent of intergranular fracture decreased, indicating a decrease in the susceptibility to quench embrittlement. However, with an increase in Si content to 2 wt pct, significant grain growth occurred producing prior austenite grain sizes 2 to 3 times those observed in the base or 1 pct Si alloys. The grain growth experienced by the high Si alloys was interpreted to result from the effects the retardation of cementite nucleation and growth at austenite grain boundaries. The fatigue properties were shown to be essentially independent of cooling rate and differences in fatigue performance were assessed primarily based on a consideration of alloy additions. Fatigue crack nucleation in all samples exhibited similar characteristics, i.e. intergranular crack nucleation at a small cluster of surface grains. The larger grain sizes in the 2 wt pct Si alloys were shown to be the primary factor that affected endurance limits. The 2 wt pct Si alloys exhibited endurance limits of approximately 915 MPa while the baseline 4320 alloy and the modified 1 wt pct Si alloys exhibited higher endurance limits of approximately 1070 MPa. In comparison to fatigue data on gas carburized samples reported in the literature the samples in this study exhibited more variability in measured lifetimes with many samples exhibiting runout at stress levels significantly greater than the measured endurance limits. Implications of this study with respect to the development of potential new carburizing alloys are discussed.« less

Authors:
 [1];  [1];  [2];  [2];  [3]
  1. ORNL
  2. Advanced Steel Processing and Products Research Center
  3. Colorado School of Mines, Golden
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); High Temperature Materials Laboratory
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE)
OSTI Identifier:
948053
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: SAE Transactions; Journal Volume: 116
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ALLOYS; AUSTENITE; BENDING; BORON ADDITIONS; CARBON STEELS; CEMENTITE; CRACK PROPAGATION; ELECTRON MICROSCOPY; ELECTRONS; EMBRITTLEMENT; FRACTURES; GRAIN BOUNDARIES; GRAIN GROWTH; GRAIN SIZE; HELIUM; NITROGEN; NUCLEATION; SILICON; SPECTROSCOPY; STEELS

Citation Formats

Meyer III, Harry M, Spice, Jason J, Greer, John G., Krauss, George, and Matlock, D. K.. Effects of Silicon and Boron Additions on the Susceptibility to Quench Embrittlement and the Bending Fatigue Performance of Vacuum Carburized Modified 4320 Steel. United States: N. p., 2007. Web.
Meyer III, Harry M, Spice, Jason J, Greer, John G., Krauss, George, & Matlock, D. K.. Effects of Silicon and Boron Additions on the Susceptibility to Quench Embrittlement and the Bending Fatigue Performance of Vacuum Carburized Modified 4320 Steel. United States.
Meyer III, Harry M, Spice, Jason J, Greer, John G., Krauss, George, and Matlock, D. K.. Mon . "Effects of Silicon and Boron Additions on the Susceptibility to Quench Embrittlement and the Bending Fatigue Performance of Vacuum Carburized Modified 4320 Steel". United States. doi:.
@article{osti_948053,
title = {Effects of Silicon and Boron Additions on the Susceptibility to Quench Embrittlement and the Bending Fatigue Performance of Vacuum Carburized Modified 4320 Steel},
author = {Meyer III, Harry M and Spice, Jason J and Greer, John G. and Krauss, George and Matlock, D. K.},
abstractNote = {The potential to reduce the susceptibility to quench embrittlement, a fracture mechanism that promotes intergranular fracture in high carbon steels, and to improve the bending fatigue performance of vacuum carburized modified SAE 4320 steels was evaluated. Data were obtained on an industrially produced SAE 4320 steel and four laboratory produced steels based on the 4320 composition but with additions of Si (1.0 or 2.0 wt pct) and B (0 or 17 ppm). All five alloys were vacuum carburized together and gas quenched with three different cooling rates as controlled by the gas quench conditions: 10 bar nitrogen, and 15 and 20 bar helium. Modified Brugger fatigue samples of each alloy and quench condition were tested in cantilever bending and failed samples were analyzed with scanning electron and Auger spectroscopy. Standard S-N curves and endurance limits were obtained and the fracture surfaces were evaluated using both light and electron microscopy techniques to determine fracture initiation sites and fracture growth mechanisms, both in the stable fatigue crack growth zone and in the overload zone. The percentage of transgranular fracture in the carburized case was quantified and used as a measure of the susceptibility to quench embrittlement. The susceptibility to quench embrittlement was observed to be independent of quench rate and boron additions, but depended on Si content. With an increase in Si content, the extent of intergranular fracture decreased, indicating a decrease in the susceptibility to quench embrittlement. However, with an increase in Si content to 2 wt pct, significant grain growth occurred producing prior austenite grain sizes 2 to 3 times those observed in the base or 1 pct Si alloys. The grain growth experienced by the high Si alloys was interpreted to result from the effects the retardation of cementite nucleation and growth at austenite grain boundaries. The fatigue properties were shown to be essentially independent of cooling rate and differences in fatigue performance were assessed primarily based on a consideration of alloy additions. Fatigue crack nucleation in all samples exhibited similar characteristics, i.e. intergranular crack nucleation at a small cluster of surface grains. The larger grain sizes in the 2 wt pct Si alloys were shown to be the primary factor that affected endurance limits. The 2 wt pct Si alloys exhibited endurance limits of approximately 915 MPa while the baseline 4320 alloy and the modified 1 wt pct Si alloys exhibited higher endurance limits of approximately 1070 MPa. In comparison to fatigue data on gas carburized samples reported in the literature the samples in this study exhibited more variability in measured lifetimes with many samples exhibiting runout at stress levels significantly greater than the measured endurance limits. Implications of this study with respect to the development of potential new carburizing alloys are discussed.},
doi = {},
journal = {SAE Transactions},
number = ,
volume = 116,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • Phosphorus and carbon segregation to austenite grain boundaries and its effects on fatigue and fracture were studied in carburized modified 4320 steel with systematic variations, 0.005, 0.017, and 0.031 wt pct, in alloy phosphorus concentration. Specimens subjected to bending fatigue were characterized by light metallography, X-ray analyses for retained austenite and residual stress measurements, and scanning electron microscopy (SEM) of fracture surfaces. Scanning Auger electron spectroscopy (AES) was used to determine intergranular concentrations of phosphorus and carbon. The degree of phosphorus segregation is directly dependent on alloy phosphorus and carbon content. The degree of carbon segregation, in the form ofmore » cementite, at austenite grain boundaries was found to be a function of alloy phosphorus concentration. The endurance limit and fracture toughness decreased slightly when alloy phosphorus concentration was increased from 0.005 to 0.017 wt pct. Between 0.017 and 0.031 wt pct phosphorus, the endurance limit and fracture toughness decreased substantially. Other effects related to increasing alloy phosphorus concentration include increased case carbon concentration, decreased case retained austenite, increased case compressive residual stresses, and increased case hardness. All of these results are consistent with the phosphorus-enhanced formation of intergranular cementite and a decrease in carbon solubility in intragranular austenite with increasing phosphorus concentration. Differences in fatigue and fracture correlate with the degree of cementite coverage on the austenite grain boundaries and the buildup of phosphorus at cementite/matrix interfaces because of the insolubility of phosphorus in cementite.« less
  • The effect of different stress states on the stress-induced martensitic transformation of retained austenite was investigated in carburized 4320 steels with an initial retained austenite content of 15 pct. Experiments were conducted utilizing a specialized pressure rig and comparison between stress-strain behaviors of specimens with different austenitization and tempering histories was performed under these stress states. Experimental results indicated considerable asymmetry between tension and compression, with triaxial stress states resulting in the highest strength levels for the untempered material. Fine carbide precipitates due to low-temperature tempering increased the strength and ductility of the specimens and also changed the austenite-to-martensite transformationmore » behavior. Numerical simulations of stress-strain behaviors under different stress states were obtained, with an existing micromechanical self-consistent framework utilizing the crystallographic theory of austenite/martensite transformation and the minimum complementary free-energy principle. The model was modified for carburized steels upon microstructural investigation and predicted the same trends in effective stress-effective strain behavior as observed experimentally.« less
  • The bending fatigue performance of aircraft-quality AISI 8620 steel was evaluated after vacuum carburizing. Case depths of 0.45 mm (0.018 in.) and 1.30 mm (0.051 in.) were produced by carburizing in propane atmospheres, and both sets of specimens were subjected to a cold-stabilizing treatment at {minus}73 C ({minus}100 F) before tempering. Bending fatigue specimens were characterized by light metallography to determine microstructure and prior austenitic 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 microstructure of the specimens with the deeper case contained 10--20{micro}mmore » diameter carbides at the prior austenitic grain boundaries, and had a fatigue endurance limit of 1,230 MPa (180 ksi). The shallow-case specimens had case microstructures with 1--2{micro}m diameter spherodized carbides decorating the austenitic grain boundaries, and had an endurance limit of 1,370 MPa (200 ksi). The high endurance limits resulting from both carburizing processes are attributed to the lack of surface intergranular oxidation and to high residual compressive surface stresses, in excess of 900 MPa (130 ksi), which are attributed primarily to the cold-stabilization treatment.« less
  • This paper explores the consequences and analyzes the kinetics of the intergranular corrosion of the Cr-depleted zones in sensitized stainless steel. The corrosive attack of the Cr-depleted zones generates occluded grooves along the grain boundaries (g.b.) which are unable to passivate under the prevailing potential. The g.b. grooves deepen and branch beneath the surface until they eventually perforate the steel membrane. Furthermore, the resulting local electrode potentials within the g.b. grooves allow for hydrogen evolution even though the outer surfaces of the steel are held at much more oxidizing electrode potentials which prohibit this reaction. Using a Devanathan-Stachurski cell itmore » was found that part of this ohmic (IR)-produced hydrogen dissolves into the steel. The results are explained by the IR potential shift mechanism. Similarities and differences between this anodic charging and the conventional cathodic hydrogen charging experiments are also presented.« less
  • Carbon (0.07%) steel samples containing about 0.04% Nb singly and in combination with nitrogen were carburized in a natural Titas gas atmosphere at a temperature of 1,223 K (950 C) and a pressure of about 0.10 MPa for 1/2 to 4 h, followed by slow cooling in the furnace. Their microstructures were studied by optical microscopy. The austenite grain size of the case and the case depths were determined on baseline samples of low-carbon steels and also on niobium and (Nb + N) microalloyed steel samples. It was found that, when compared to the baseline steel, niobium alone or inmore » combination with nitrogen decreased the thickness of cementite network near the surface of the carburized case of the steels. However, niobium in combination with nitrogen was more effective than niobium in reducing the thickness of cementite network. Niobium with or without nitrogen inhibited the formation of Widmanstatten cementite plates at grain boundaries and within the grains near the surface in the hypereutectoid zone of the case. It was also revealed that, when compared to the baseline steel, niobium decreased the case depth of the carburized steels, but that niobium with nitrogen is more effective than niobium along in reducing the case depth. Niobium as niobium carbide (NbC) and niobium in the presence of nitrogen as niobium carbonitride, [Nb(C,N)] particles refined the austenite grain size of the carburized case, but Nb(C,N) was more effective than NbC in inhibiting austenite grain growth.« less