Iron-copper-carbon Steels

Presentedat World Congress PM2014 in Orlando USA on May 19, 2014 Page 1 CHARACTERIZATION OF NICKEL ADDITIONS TO HEAT TREATED IRON-COPPER-CARBON MATERIALS Amber Neilan, Sarah Ropar, Bo Hu, & Roland Warzel III North American Hoganas 111 Hoganas Way Hollsopple, PA 15935 ABSTRACT Iron-copper-carbon steels are the most prevalent material system used in powdered metal (PM) components.Nickel is a common alloying element used to improve heat treatment response and increase ductility. Understanding the effects of nickel additions after heat treatment will provide component manufacturers the ability to select the proper material system for a given set of requirements. This study looks at characterizing the use of nickel in iron-copper-carbon heat treated materials.The mechanical performance and metallography of heat treated materials with varying additions of nickel will be presented. INTRODUCTION Alloying elements are commonly added to ferrous systems to improve heat treatment response and amplify mechanical properties. Copper and nickel are two popular alloying elements commonly used in the PM industry to strengthen and harden iron steels. In order to enhance part performance, it is important to understand how the individual effects of copper and nickel in heat treated materials can be altered when they are combined together. Presentedat World Congress PM2014 in Orlando USA on May 19, 2014 Page 2 Figure 1: SEM photographs of the particle sizes of copper and nickel Copper is highly desired alloy addition for PM materials due to its strength characteristics. The typical copper particle size is approximately 80m, while nickel is much smaller at 10 m (Figure 1) [1]. Copper melts at 1083 C (1981 F), which is below conventional sintering temperatures. The low melting temperature allows even coarse copper particle sizes to liquefy and diffuse completely into the iron. During sintering, copper in its molten form diffuses along the grain boundaries of iron and results in swelling [2]. Studies have shown that when used in conjunction with carbon, a reaction between the two results in a decrease in swelling [3].By using copper and carbon additions, iron based materials provide desired dimensional control and mechanical properties for application requirements. In the heat treated form, copper has little effect on strengthening properties so carbon additions are generally used to provide the added strength [1] [4]. Nickel is also a popular choice among alloying elements in iron-carbon steels. Nickel is used for achieving high strength, high impact resistance, and high hardenability. Nickel provides added ductility to materials and significantly improves hardenability and strength properties. Unlike copper, nickel results in shrinkage of materials and provides dimensional stability after sintering. The response nickel has to heat treatment is also a desirable aspect as it provides even higher strengthening properties [5]. The melting point of nickel is higher than conventional sintering temperatures, causing it to alloy to iron by solid-state diffusion [5].This causes the nickel to only partially diffuse into the iron, creating nickel rich austenitic areas. When quenched, the nickel rich areas stabilize austenite, while the remaining partially alloyed iron transforms into martensite. The austenite that is formed during heat treatment contributes to the ductile nature of heat treated nickel alloyed materials. This ductility allows higher strength values and fracture resistance to be obtained. Heat treatment is defined as the process of heating a material above its austenization temperature and cooling at a specific rate in order to obtain specific properties [6]. Heat treatment is an important process in the PM industry because it allows parts manufacturers to improve both the static and dynamic properties of material systems. By using a specifically designed material system, properties such as wear resistance, strength and hardenability can be optimized using a secondary heat treatment process. Tempering is commonly used after heat treatment processes as a method of relieving residual stresses that accompany the non-equilibrium cooling or quenching after heat treat [7]. The brittle nature of heat treated materials makes tempering a necessary step when looking for strengthening enhancement. Tempering is a Presentedat World Congress PM2014 in Orlando USA on May 19, 2014 Page 3 method of heating a previously hardened component below the materials critical temperature for a certain time, followed by a slow cooling. Tempering provides added ductility and toughness, while also reducing distortion of the previously hardened material [7]. There are a wide range of heat treat procedures that can be utilized and tailored for specific applications. This paper focuses on conventional quench and temper heat treatment of alloyed iron carbon steels. The reaction to heat treatment of various copper and nickel additions will be examined to determine the effect each alloy element contributes to mechanical properties. EXPERIMENTAL PROCEDURE Seven compositions of Fe-C steels with various copper and nickel additions were prepared (The compositions are shown in Table 1). Two levels of carbon (0.60% and 0.80%) were targeted for each of those compositions for a total of fourteen mixes. All mixes used the same base iron (North American Hgans ASC100.29), copper (Cu-165 ACu Powder International, LLC.), nickel (Vale Inco Ni 123), graphite (Asbury Graphite Mills 1651) and lubricant (Intralube E, Hgans AB). Table 1: Material Systems Evaluated (wt %) Mix ID CuNi Combined Alloy Addition (%) 12-2 21.50.52 3112 40.51.52 5-22 6213 7123 Each mix was compacted into standard TRS bars, tensile bars, and impact energy bars at varied green densities with the objective of reaching a sintered density of 6.9 g/cm3. The bars were sintered in a mesh belt furnace at 1120 C (2050 F) for 20 minutes in a 90% nitrogen / 10% hydrogen atmosphere. Once sintered, the specimens were sent to a certified commercial facility for heat treatment (Advanced Heat Treat, Inc. -St. Marys, PA). The bars were austenitized at 843 C (1550 F) in air for 1 hour with a 0.8% carbon potential. The specimens were then oil quenched and tempered at 204 C (400 F) for 1 hour in air. After heat treatment, the TRS bars were evaluated for hardness, dimensional change, sintered carbon, and sintered density.The tensile bars and impact bars were sent to a certified testing laboratory for evaluation of ultimate tensile strength and impact energy (Westmoreland Mechanical Testing and Research, Inc. -Youngstown, PA). Metallographic analysis was also performed on selected specimens. The fracture surface after the impact energy tests was examined using a Scanning Electron Microscope (SEM). All testing was done in accordance with MPIF standards [4]. RESULTS The effect of nickel and copper additions on tensile strength at each sintered carbon level is shown in Figure 2.Presentedat World Congress PM2014 in Orlando USA on May 19, 2014 Page 4 Figure 2: Ultimate tensile strength of Fe-C steels with various nickel and copper contents at 6.9 g/cm3 A linear decrease in strength is observed in both the high and low carbon material systems as the nickel content decreases. The low carbon materials show a decrease in strength over the 0% copper to 1% copper range as the nickel content decreases. After the 1% copper range, the strength begins to increase. A 25 MPa difference is seen between the Fe-Ni and Fe-Cu materials. By increasing the sintered carbon content to 0.80%, a constant decrease in strength is seen throughout the whole copper-nickel range. A 50 MPa difference is seen from the 2% nickel material compared to the 2% copper material. The overall strength of the high carbon materials is significantly lower than the strength of the low carbon materials. The highest strength observed was in the case of the 0.60% carbon iron-nickel material with no copper addition.The dimensional change at each carbon level is shown in Figure 3.Figure 3: Dimensional change of Fe-C steels with various nickel and copper contents at 6.9 g/cm3 All material systems increase in growth as the nickel content decreases. For the low carbon system, the dimensional change increases linearly for the entire copper range, however the growth is not as significant from the 1.5% copper to the 2% copper amount.At the higher carbon level the dimensional change increases linearly between the 0% copper and 1.5% copper range. Above 1.5% copper, the dimensional change begins to decrease. Presentedat World Congress PM2014 in Orlando USA on May 19, 2014 Page 5 DISCUSSION Metallographic analysis was performed in order to understand the response of the copper-nickel material systems after processing. The microstructures of materials with 2% copper, 1% copper with 1% nickel, and 2% nickel are shown in Figure 4. Figure 4: Microstructure of heat treated Fe+0.6%C steels with different copper and nickel additions

The heat treatment resulted in the formation of martensite. An increase in the nickel content resulted in an increase of localized nickel-rich austenite. With 0.60% carbon, the iron copper material is martensitic with little observed retained austenite. Looking at the 1% nickel, 1% copper material in Figure 4, nickel-rich austenite begins to form along the grain boundaries where the nickel alloying content is higher. The material containing 2% nickel has the highest amount of nickel rich austenite. Relating the austenite formation with the strength, the highest strength is observed in the material containing 2% nickel with the localized austenite. The formation of nickel rich austenite provides an inherent ductility to materials. The added ductility aids in a materials resistance to premature fracture that is seen in brittle, fully martensitic systems.The lowest strength is observed in the 2% Cu materials. This is the result of austenite formation due to carbon that forms between the martensite, causing brittleness. Similar phase patterns are seen in the 0.80% sintered carbon material systems. The Fe-Cu material demonstrates a predominantly martensitic microstructure. The addition of nickel increases the formation of the localized nickel-rich austenite. The strength values also follow trend, where the fully martensitic Fe-Cu results in the lowest strength.The strength increases as the balance of martensite and austenite increases. The microstructures comparing carbon content in the Fe-Cu-C materials vs. the Fe-Ni-C materials are shown in Figure 5. Presentedat World Congress PM2014 in Orlando USA on May 19, 2014 Page 6 Figure 5: Microstructure of Fe-Cu vs. Fe-Ni materials at high and low carbon levels The difference in carbon content between the two iron-copper and iron-nickel materials results in a distinction between the thickness of the martensite needles. The low carbon material shows a lath martensite, with fine martensitic needles. The high carbon material demonstrates a plate martensite, with dense, coarse needles. The density and texture of the martensite plays a role in the strength values. The lath martensite observed in the low carbon materials resulted in high strength results. The plate martensite obtained in the high carbon materials promotes brittleness, resulting in early fracture and low strength. Examining the difference of phases over the spectrum of copper-nickel additions, the combination of high amounts of nickel rich austenite and finely dispersed martensite that is seen in the high nickel, low carbon iron material provides optimum strength values in comparison to the varying alloy additions of the other low carbon materials. The 0.80% carbon Fe-Cu material that exhibits a coarse, dense, fully martensitic structure falls on the other end of the strength spectrum, presenting the lowest strength values due to the brittleness of the material. Presentedat World Congress PM2014 in Orlando USA on May 19, 2014 Page 7 Figure 6: SEM analysis of martensite on iron-copper material at two different carbon levels. The SEM photographs depict the difference in martensite. The material with 0.6% carbon has fine, lath martensite while the material with 0.8% carbon has coarse, plate martensite. The microhardness values are shown in Figure 7. Figure 7: Microhardness of various nickel and copper contents The microhardness of the materials with 0.80% carbon linearly decreases as the nickel content decreases. The Fe-2Ni material exhibits the highest microhardness, while the Fe-2Cu material presents much lower microhardness. The microhardness of the 0.6% carbon materials is similar for the F-2Cu and Fe-2Ni materials. Comparing results with the microstructures, the Fe-2Cu material contains a high amount of austenite due to carbon diffused throughout the martensite, resulting in lower microhardness. For the Fe-2Ni materials, the nickel rich austenite remains localized around the areas of nickel diffusion. Apparent hardness was similar for all materials at each carbon level. The fracture surface of the Fe-2Cu and Fe-2Ni materials with 0.6% sintered carbon is shown in Figure 8. Presentedat World Congress PM2014 in Orlando USA on May 19, 2014 Page 8 Figure 8: SEM photographs of fracture surfaces of iron copper and iron nickel materials at two carbon levels At the low sintered carbon level, the fracture surface of both the Fe-2Cu and Fe-2Ni materials exhibit cup-cone fractures with some shear fractures. The cup-cone fractures are indicative of a ductile break where the material slowly pulls away from each other, causing large amounts of deformation before failure [8]. In cup-cone fractures, the surface appears dimpled. Shear fractures suggest brittle failure, where the material breaks with very little plastic deformation before fracturing.In brittle fracture, the surface appears flat and smooth. Comparing the fracture surfaces of both carbon levels, the high carbon materials exhibit a higher percentage of shear fractures than the low carbon materials. The dimple fractures appear large and shallow, with less deformation observed. The low carbon materials have a larger quantity of deep, open dimples with high deformation. The low carbon nickel material exhibits the least amount of shear fractures, and contains the highly deformed cup-cone fractures. The high carbon, iron copper material exhibits the largest number of shear fractures and contains shallow dimples with little deformation before fracture. Relating the fracture surfaces to the data, the highest strength was observed in the low carbon iron nickel material, and has the observed ductility in the fracture surface. The lowest strength measured was the high carbon copper material, which has the observed shear, i.e. brittle fractures on the surface.

Presentedat World Congress PM2014 in Orlando USA on May 19, 2014 Page 9 After looking into the 2% total combined alloy materials, the alloy content was increased to 3% in order to determine if higher combined alloy content provides any added benefit to strength or dimensional control. The strengths of the 3% combined alloy materials are shown in Figure 9 to compare with the 2% combined alloy materials. Figure 9: Strengths of 3% Combined Alloy Materials (Red) verses 2% Total Combined Alloy Materials (Blue) With a total 3% combined alloy content, the material with the higher nickel addition results in higher strengths at both sintered carbon levels than the material with lower nickel additions. The 0.6% sintered carbon has the highest overall strength for both alloy compositions than the 0.8% carbon results. Comparing the results to the 2% combined alloy addition, however no increase in strength is observed by increasing the total alloy composition to 3%. In fact, the overall strengths of the 3% alloy additions are lower than the 2% alloy addition strengths at both carbon levels.

The dimensional change of the 3% combined alloy materials are shown in Figure 10 to compare with the 2% combined alloy materials. Figure 10: Dimensional Change of 3% Combined Alloy Materials (Red) verses 2% Total Combined Alloy Materials (Blue) Looking at the dimensional change of the 3% combined alloy additions, a significant jump in growth is observed in the material containing 2% Cu and 1% Ni at both carbon levels. The dimensional change observed is approximately 0.15% higher than the largest growth observed in any of the 2% alloy materials. No benefit in dimensional change is seen in the 3% combined alloy materials at both carbon Presentedat World Congress PM2014 in Orlando USA on May 19, 2014 Page 10 levels compared to the 2% combined alloy materials.The large jump in dimensional change in the 3% combined alloy additions was also observed in the as-sintered condition. CONCLUSION The strength of the heat treated Fe-Cu- C materials increases as the addition of nickel increases in both the high and low carbon levels. The strength decreases as the carbon increases. For low carbon materials, the increase in nickel additions reduces growth. For the high carbon materials, at least 1% nickel is needed to obtain added shrinkage. The strength of the materials with 1% Cu and 2% nickel are higher than the materials with 2% Cu and 1% Ni at both carbon levels. The 3% combined alloy materials however have no significant increase in strengths compared to the 2% combined alloy materials. The additional alloy content provides no benefit to strength. For the 3% combined alloy materials, the increase in nickel of provides added shrinkage compared to the higher copper material at both carbon levels. However, adding 1% nickel into Fe-2Cu materials provides further growth rather than shrinkage. The additional alloy content provides no benefit to dimensional control. REFERENCES [1]Singh, T., Stephenson, T., & Campbell, S. (n.d.). Nickel-Copper Interactions in P/M Steels. [2]Copper Developement Association, I. (2013). Retrieved from Copper in Iron and Steel P/M Parts: http://www.copper.org/resources/properties/129_6/copper_iron.html [3]J onnalagadda, Krishnapraveen (n.d) Influence of Graphite Type on Copper Diffusion In Fe-Cu-C PM Alloys (pg.11).[4]MPIF Standard 35 Material Standards for PM Structural Parts. (n.d.). MPIF [5]Steel Heat Treatment: Metallurgy and Technologies/Totten, George E. Editor. (2007) CRC Press Taylor & Francis Group (pg.760). [6]Steel Heat Treatment Handbook/Totten, George E., Howes, Maurice A. Editors (1997) Marcel Dekker Inc.[7]Dossett, J . L., & Boyer, H. E. (2006). Practical Heat Treating (2 ed.). ASM International. (pgs 5-6). [8]German, R. M. (2005). Powder Metallurgy & Particlulate Materials Processing. MPIF