Influence of Tempering Parameters on Microstructure, Mechanical Properties of Low-Alloy PM Steels
Tempering is a heat-treatment technique used to improve the strength, ductility and toughness of hardened carbon steels.
Tempering occurs by heating the steel to below its critical temperature in order to transform the metastable body-centered tetragonal martensite structure that is formed during quenching into a more stable structure of fine carbide particles. Choosing the correct tempering parameters is critical to achieving the desired balance of properties.
It is important to understand the role tempering has on strength and toughness properties, as well as the microstructural changes experienced over a range of tempering temperatures. This study examined the mechanical performance of two different alloy systems – pre-alloyed and diffusion-bonded material – after being heat treated and tempered at various tempering parameters.
The production of powdered-metal (PM) components often requires a secondary heat-treat process in order to meet the strength and hardness requirements of high-performance applications. While there are various heat-treatment processes utilized for PM components, case hardening and through hardening are two common methods used throughout the industry.
Case hardening, also known as case carburizing, is a process in which low-carbon steels are heated to their austenitizing temperature, typically from 850-950°C (1560-1740°F), in a carbon-rich-atmosphere environment. Due to the high solubility of carbon in austenite, the carbon is absorbed at the surface of the component. This high-carbon layer is then quenched to form a martensitic case, typically in an agitated-oil medium.
The purpose of this method is to develop a case with good surface hardness while preserving the relatively soft but tough inner core. Case hardening is used for components that require high surface-wear capabilities, good fatigue life and shock-load resistance. Common PM case-hardened applications include gears and sprockets.
Through hardening is a heat-treatment method typically reserved for components that will be exposed to axial stresses and that do not require particularly high surface hardness.
Through hardening is typically conducted using a quench-and-temper process where the component is heated to its austenitizing temperature in a carbon-neutral atmosphere, then rapidly quenched in a medium such as agitated oil.
Unlike case hardening, through hardening utilizes the carbon within the material for hardening, with the goal of martensite formation throughout the whole component and not just at the surface.
The component size and geometry play a significant role in the overall amount of martensite developed in through-hardened components. The cooling rate for any component is directed by thermal conduction. For large components with thick cross sections, the surface of the component will be subject to a different cooling rate compared to the inner core, where the cooling rate is limited. This limitation of cooling in the core may hinder the development of a fully martensitic component. In comparison, thin cross-sectional areas will see more consistent and faster cooling throughout the section, leading to easier transformation to fully martensitic components.
Heat Treatment and Temper
The purpose of any type of heat treatment is to improve the mechanical properties by changing the microstructure to martensite, which contributes to high strength and high hardness properties. There is insufficient time for carbon atoms to precipitate out of the crystal lattice during rapid cooling from the austenitizing temperature. This causes the face-centered cubic (FCC) crystal structure of austenite to transform into a supersaturated solid solution of carbon trapped in a body-centered tetragonal (BCT) structure. This structure is known as martensite.
The rapid transformation introduces a large amount of dislocations within the crystal structure, which causes high levels of internal stress. This stress results in a very hard but extremely brittle material. In order to relieve the stresses, tempering is used as a technique to restore some ductility and toughness back into the material while giving up some hardness.
Tempering occurs when a material is reheated to a temperature below its eutectoid point for a specified amount of time, allowing for the rearrangement of atoms and precipitation of carbon to relieve the internal stresses and modify the martensitic structure. During the tempering phase, the rearrangement of atoms and the precipitation of carbon results in the arrangement of spherical carbides being dispersed within ferrite. This arrangement is known as a tempered martensite structure. Tempering for PM steels is typically carried out at temperatures between 150-595°C (302-1103°F). Precise control of time and temperature during the tempering phase is important to achieve the desired final mechanical performance.
The hardenability of a material also plays an important role in achieving the desired properties. Hardenability is defined as the ability of a material to achieve a certain hardness level at a given depth after heat treatment. It is a measure of how easily a material will form martensite and the depth at which martensite will develop when quenched.
High-hardenability materials will form martensite not only at the surface but also throughout the core of a component. The depth of hardening is an important factor in a part’s toughness and is largely influenced by the material’s carbon level and alloying elements. The most common alloying elements used in the PM industry are molybdenum, nickel, manganese, chromium and copper. The effect of various alloying elements and additions on the hardenability of a material are shown in Figure 1.
The diagram depicts a multiplication factor that describes the depth of hardening when adding a certain quantity of the alloying element. As seen from the diagram, molybdenum, chromium and manganese have a strong influence on a materials hardenability, while the influence of nickel is much less. The choice of an alloying element is directed by the alloy’s ability to raise the hardenability of the material, the quantity needed and also its ability to diffuse consistently throughout the material.
Regardless of the type of heat treatment a PM component receives, the material alloying method plays a significant role in the final properties. In pre-alloyed systems, addition of alloying elements during the melting process creates a chemically homogenous alloyed particle. Due to the homogeneity of the alloying content, a pre-alloyed material system typically generates a homogenous microstructure in the as-sintered phase.
An alternative method to introduce alloying elements is through a diffusion-bonding process. In this process, the alloying elements are thermally bonded to the surface of the iron particle. This method provides the benefit of having the alloy additions without compromising the soft, easily compressible iron-particle core.
Because the diffusion-bonded materials are not one homogenous chemistry like a pre-alloyed material is, however, the as-sintered microstructural formation is heterogeneous with various island-like phases, which are dependent on the specific alloying element present in the area. In the heat-treated form, pre-alloyed and diffusion-bonded materials are both capable of achieving a martensitic microstructure. Figure 2 depicts the microstructures of a pre-alloyed (a) and diffusion-alloyed (b) base iron in the as-sintered condition.
In this study, two alloy systems with similar chemical identities – one pre-alloyed and one diffusion-bonded – were through hardened and tempered at various tempering parameters after conventional sintering. The purpose of this study is to investigate the response of alloying method and component size to heat-treat and tempering conditions and to determine how the tempering temperature affects the mechanical properties.
One commercially available pre-alloyed base iron and one diffusion-alloyed base iron commonly used in heat-treatment applications were chosen for this study. The Astaloy® 85 Mo is pre-alloyed, while the D.AB is diffusion-bonded base iron. The alloy compositions are listed in Table 1.
Mixes were manufactured from the two alloys according to MPIF material designations. In the FLN2-4405 mix, the nickel is admixed in. In the FD-0205 mix, no additional alloying elements were added. Both mixes contain the same type and amount of graphite and lubricant. The chemical compositions are shown in Table 2.
Each mix was compacted into 10-mm x 10-mm x 75-mm (0.39-inch x 0.39-inch x 2.95-inch) specimens to a green density of 7.25 g/cm3. They were conventionally sintered at Vision Quality Components in Clearfield, Pa. All of the specimens were heat treated at Bluewater Thermal Solutions in St. Marys, Pa. The heat-treatment parameters are shown in Table 3. All sintered and heat-treated specimens were prepared for mechanical testing in accordance with MPIF standards.
The heat-treated specimens were then tempered for 1 hour in air at various temperatures ranging from 160-275°C (325-525°F). The heat-treated and tempered specimens were evaluated for ultimate tensile strength, yield strength, impact energy, apparent hardness, microhardness and microstructure.
A secondary evaluation for mass effect was conducted on two different-size puck specimens manufactured from the FLN2-4405 material. The mix was compacted into 100-mm-diameter x 25-mm-high (4-inch x 1-inch) pucks, and 40-mm-diameter x 25-mm-high (1.5-inch x 1-inch) pucks, respectively.
The puck specimens were all compacted to a green density of 7.25 g/cm3. The specimens were sintered in a 6-inch laboratory belt furnace at 1120°C (2050°F) for 30 minutes in a 90/10 N2/H2 atmosphere. The sintered pucks were then heat treated at Bluewater Thermal Solutions at the same parameters as shown in Table 3. The heat-treated pucks were tempered at the same temperatures used for the tensile and impact specimens.
One heat-treated puck without tempering was also evaluated. The pucks were evaluated for apparent hardness, microhardness profile and microstructure. Phase mapping was completed on the 100-mm puck specimen tempered at 200°C (400°F) to determine the percentage of martensite at incremental distances from the part surface.
The tensile strength at each tempering temperature is shown in Figure 3. The pre-alloyed FLN2-4405 material resulted in overall higher tensile strengths at each tempering temperature compared to the diffusion-bonded FD-0205 material. The tensile strength for each material decreases as tempering temperature increases. Both material systems follow a similar trend, where a sharp decline in tensile strength is observed as the tempering temperature reaches beyond 220°C (425°F).
The yield strength at each tempering temperature is shown in Figure 4. The pre-alloyed FLN2-4405 material resulted in overall higher yield strengths at each tempering temperature compared to the diffusion bonded FD-0205 material. The yield strength for both materials increases as tempering temperature increases until the temperature reaches 250°C (475°F). When the temperature exceeded 250°C (475°F), the yield strength began decreasing.
The impact energy at each tempering temperature is shown in Figure 5. The impact energy of both material systems is similar within the tempering range of 160-200°C (325-400°F). As the tempering temperature increased above 200°C (400°F), the impact energy began to drop significantly with each tempering-temperature increment.
The apparent hardness is shown in Figure 6. Higher apparent-hardness levels at all tempering temperatures were obtained from the pre-alloyed FLN2-4405 material when compared to the diffusion-bonded FD-0205 material. The apparent hardness decreases as tempering temperature increases for both materials.
The microhardness is shown in Figure 7. The microhardness between the FLN2-4405 and FD-0205 material systems was similar at each tempering temperature. The microhardness levels decreased as tempering temperature increased.
In the mass-effect study, a microhardness profile on the martensite was measured on the FLN2-4405 specimens. The profile was developed at 1-mm increments into the core of the components (Fig. 8). The microhardness decreased at each 1-mm increment below the surface. The microhardness also decreased as the tempering temperature increased. The untempered pucks resulted in the highest microhardness due to stresses developed on the matrix by the heat treatment.
We will discuss the results and conclude the summary of our work in June.
For more information: Contact Amber Tims, Technical Service Engineer, North American Höganäs Co. 111 Hoganas Way, Hollsopple, PA 15935; tel: 814-479-3528; e-mail: email@example.com; web: www.hoganas.com.
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