Quantitative Metallography

Quantitative Metallography Technical Publications

197. Metallographic Analysis of PM Fracture Surfaces The fracture surfaces produced by breaking PM materials appear substantially different from those generated on parts made by other metalworking techniques. Although the characteristics of the fractured regions are the same, ductile dimples, cleavage, etc., the interior pore surfaces add complications to a fractographic analysis. The smooth surfaces of the inherent porosity are not only common with PM materials, they are expected. Part density, the effectiveness of sintering, and the history of the part in the green state have large effects on the microstructure and subsequently, on the appearance of the broken surfaces. The fraction of pore edges, which appear as smooth free particle surfaces, varies with density, where low-density regions display large amounts of these free surfaces. Sintering causes the particles forced into intimate contact during compaction to form metallurgical bonds. Better sintering results in neck growth and a more homogeneous distribution of alloying elements. Inappropriate handling of green parts or compaction/ejection problems can create internal cracks within the parts prior to sintering.

Techniques will be presented both to prepare surfaces for analysis and to quantify the appearance of fracture surfaces. Examples of PM materials broken using different types of loading will be used to demonstrate the use of these methods. In addition, an investigation using an FC-0205 material was conducted to investigate the effects of the three variables mentioned above. Test pieces were compacted to several densities and sintered using various temperatures to produce density and sinter quality variations. In addition, cracks were introduced into green test pieces and examined after sintering. SEM analysis of the fractures was used in concert with light microscopy on prepared cross-sections to evaluate and quantify the resulting fracture surfaces.

178. Effects of Residual Surface Stress and Tempering on the Fatigue Behavior of Ancorsteel 4300 In this study, the microstructure, residual stress and fatigue behavior of Hoeganaes’ Ancorsteel 4300 sintered steel were characterized. Samples were tempered at either 205°C or 315°C and then machined by low-stress grinding. The effects of residual surface stress (measured by XRD) on the fatigue behavior were studied by either application of a stress-relieving heat-treatment after machining or by polishing away the machined surface. Based on this study, samples tempered at 315°C had slightly higher fatigue strengths than samples tempered at 205°C. Although significant compressive stresses were induced by machining on the surface of samples, these residual stresses did not affect the fatigue behavior. Rather, the fatigue behavior was controlled by porosity inherent to these sintered steels.

174. Fatigue Crack Growth of Prealloy Fe-0.85Mo-2Ni-0.6C Steels with a Homogeneous Microstructure The fatigue crack growth behavior of powder metallurgy steels (P/M steels) is strongly affected by the nature of porosity and microstructure of steel matrix. Our previous work has focused on a premix P/M steel prepared from Fe-0.85Mo prealloy mixed and binder-treated with 2%Ni and 0.6% graphite. In this study, we have studied the fatigue crack growth behavior of a prealloy steel of similar composition. Use of the prealloy powder resulted in more homogenous microstructure than the premix steel. The alloys were tested at three different densities: 7.0 g/cm³, 7.3 g/cm³, and 7.5 g/cm³. Microstructure characterization was conducted by optical and scanning electron microscopy (SEM). Fatigue testing was performed at various R-ratios, ranging from –2 to 0.8. Increasing porosity and increasing R-ratio resulted in a decrease in ∆K_th. The degree of crack closure was measured for both premix and prealloy steels at different R-ratios, and is discussed.

157. Quantifying the Degree-of-Sinter in Ferrous P/M Materials Improvements in the physical and mechanical properties of pressed and sintered ferrous materials are made during the sintering process. Particle bonding and alloying by diffusion occur during sintering with property enhancements resulting as the sintering time is increased. The effects of sintering are visible as changes in microstructural features, such as particle boundaries and pore edges. Some of the improvements in sintering appear as a loss in particle boundaries, smoother pore edges, and a lessening in the number of angular features between particles. The appearance of these features and characteristics, in conjunction with their frequency of occurrence, is often referred to as degree-of-sinter. Quantification of the degree-of-sinter can be performed on properly prepared metallographic specimens using well-understood stereological practices. Three test methods will be discussed as techniques for quantifying and separating materials sintered to varying degrees. Additionally, images of an iron-copper-carbon premix, sintered at varying times, will be used to illustrate these microstructural changes.

153. Fatigue Crack Growth of Sintered Steels with A Heterogeneous Microstructure Powder metallurgy processing of steel alloys typically results in a material with heterogeneous microstructure and residual porosity. The fatigue crack growth behavior of these materials is strongly affected by the nature of porosity and heterogeneous microstructure. Notched fatigue specimens were prepared from a Fe-0.85Mo prealloy mixed and binder-treated with 2%Ni and 0.6%C. The alloys were tested at three different densities: 6.98 g/cm^3,7.36 g/cm³, and 7.53 g/cm³. The microstructure at each density was characterized to determine the porosity, microconstituents, and phase fractions. Fatigue testing was performed at various R-ratios, ranging from –2 to 0.8. Increasing porosity and increasing R-ratio resulted in a decrease in ∆K_th. In situ observation of crack growth showed that the cracks propagated through Ni-rich regions. It appears that pearlite regions, and, to some extent bainite regions, contributed to toughening and crack deflection. These findings are supported by quantitative measurements of crack growth rate, through the various microconstituents.

140. A Metallographic Investigation Into the Effect of Sintering on a FC-0205 Premix The properties of ferrous P/M materials are developed during the sintering process where metallurgical bonds are formed at particle-to-particle contacts and alloying of mixed and bonded additives occurs. Increasing either the sintering temperature or time can produce improvements in the microstructure and, consequently, the ensuing properties. In this paper, an FC-0205 premix, sintered for various times at 1120 °C (2050 °F), will be used to study the microstructural changes resulting from increases in sintering time. Features of interest include, changes to the surface-to-volume ratios at the particle boundary and pore surface areas, diffusion of the copper in the solid state and as a liquid component, and the homogenization of the microstructrue with increasing sintering times. Stereological techniques, using light optical microscopy, will be employed to examine the diffusion of the added alloying materials and to quantify the improvement in the degree-of-sinter. Additionally, electron microscopy (SEM) will be used to examine Charpy impact fracture surfaces from specimens sintered at the various times.

138. Fatigue Crack Growth of Fe-0.85Mo-2Ni-0.6C Steels with a Heterogenous Microstructure (Experiments and Microstructure-Based Simulations of the Axial Fatigue Behavior of P/M Alloys) Powder metallurgy processing of steel alloys typically results in a material with heterogeneous microstructure and residual porosity. The fatigue crack growth behavior of these materials is strongly affected by the nature of porosity and heterogeneous microstructure. Notched fatigue specimens were prepared from a Fe-0.85Mo prealloy mixed and binder-treated with 2%Ni and 0.6%C. The alloys were tested at three different densities: 6.98 g/cm³, 7.36 g/cm³, and 7.53 g/cm³. The microstructure at each density was characterized to determine the porosity, microconstituents, and phase fractions. Fatigue testing was performed at various R-ratios, ranging from –2 to 0.8. Increasing porosity and increasing R-ratio resulted in a decrease in ∆Kth. In situ observation of crack growth showed that the cracks propagated through Ni-rich regions. It appears that pearlite regions, and, to some extent bainite regions, however, contributed to toughening and crack deflection.

136. Binder Treated Products for Higher Densities and Better Precession Continuing research in the chemistry of binders and lubricants yielded novel materials that combine traditional binder properties with improved lubricity and better dimensional control. New binder-lubricant systems were developed with lower organic content that made it possible to reach higher green and sintered densities and exceptional mechanical properties. Better powder flow and higher apparent density result in more uniform die fill, giving better weight and dimensional control and increased part precision. A comparison of the newly developed binder/lubricant system is made with traditional lubricants, such as EBS and zinc stearate.

125. Effect of Density on the Microstructure and Mechanical Behavior of Powder Metallurgy FE-MO-NI Steels The microstructure and mechanical properties of Fe-0.85Mo-Ni powder metallurgy (P/M) steels were investigated as a function of sintered density. A quantitative analysis of microstructure was correlated with tensile and fatigue behavior to understand the influence of pore size, shape, and distribution on mechanical behavior. Tensile strength, Young’s modulus, strain-to-failure, and fatigue strength all increased with a decrease in porosity. The decrease in Young’s modulus with increasing porosity was predicted by analytical modeling. Two-dimensional microstructure based finite element modeling showed that the enhanced tensile and fatigue behavior of the denser steels could be attributed to smaller, more homogeneous, and more spherical porosity which resulted in more homogeneous deformation and decreased strain localization in the material. The implications of pore size, morphology, and distribution on the mechanical behavior and fracture of P/M steels is discussed.

115. Measuring Degree of Sinter Using Metallographic Methods The determination of degree-of-sinter in P/M materials historically has been difficult. Past attempts have been subjective and lacked the quantitative results that could be used in correlation studies to improve properties or monitor manufacturing processes. An attempt is made to develop a test technique using quantitative microscopy to evaluate pore structures in pressed and sintered materials. For this initial phase of the study, a F-0005 material was sintered at times ranging from 5 to 45 minutes in the heating zone. Specimens were removed, prepared by metallographic methods, and tested using quantitative microscopy techniques. The preparation procedures and results of the testing are presented along with a brief historical background.

112. Processing of Hybrid Alloys to High Densities Premixes containing prealloy molybdenum, such as Ancorsteel 85HP, nickel and graphite have exceptional mechanical properties. This presentation will highlight the properties of these materials processed to densities of 7.25 to 7.45 g/cm³ by single press, single sinter techniques. The exceptional green strength of these materials in combination with density provides a unique opportunity to convert more automotive components to P/M.

99. Effect of Microstructural InHomogeneities on The Mechanical Properties of Hybrid P/M Steels The effect of microstructural inhomogeneities on the tensile and impact response of a prealloyed (FL-4405) and two hybrid (FLC2-4405 and FLN2-4405) P/M steels was investigated. Tensile and impact response, microstructures, pore characteristics and fracture modes were determined in the sintered, quenched + tempered and sinterhardened conditions. Sintering temperatures of 1120°C (2050°F) and at 1260°C (2300°F) were utilized anddensities in the range 7.0 - 7.4 g/cm3 were achieved by single and double pressing and sintering. Over this sintered density range, tensile strength increases by >30%. In the quenched + tempered condition tensile strength exceeds 1000 MPa. Tensile properties are rationalized in terms of the attendant microstructures and modes of fracture.

75. Quantitative Image Analysis Technique for Determining Local Density Variation The PMPA Standards Committee is developing a new test method for determining the porosity of powder metallurgy products by image analysis techniques. This technique would be used to evaluate the local density variation in complex P/M parts. An inter-laboratory study was conducted to estimate the uncertainty of this new measurement technique. The results found the accuracy of the test method to be determined by test specimen preparation technique and the image analysis procedure. Such factors as magnification, number of fields examined, field-to-field variability and the detection level setting all had a role in the repeatability and reproducibility of the test method. Results are discussed for both an FC-0208 and SS-316L.

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