In order to cope with these challenges and for a reduction of alloying costs, Hoeganaes Corporation has developed a number of steel powder grades without the use of carbonyl nickel powder. They are based on Ancorsteel 50HP and Ancorsteel 85HP powders prealloyed with 0.5% and 0.8% Mo, respectively.
Two proprietary master alloys, one containing Ni; Cr and Si, the other one Mn, and natural flake graphite were admixed to these powders. Ancorbond technology was applied to firmly attach the graphite and master alloy powder particles to the steel powder with a binder. The alloy compositions were designed for sinter hardenability, resulting in properties that meet or exceed more highly alloyed diffusion alloys.
The powder grade Ancorsteel 4300 contains 0.8% Mo (prealloyed), 1.0% Ni, 1.0% Cr, 0.6% Si (master alloy) and 0.6% graphite (admixed), balance iron. The processing parameters and material properties were compared to a diffusion alloyed grade FD-0405 and a fully prealloyed Fe-Ni-Mo PM steel.
The compressibility of the 4300 grade at 550 MPa is at 7.0 g/cm³, only slightly less than the diffusion alloyed grade (7.03 g/cm³) and clearly better than the fully prealloyed grade (6.91 g/cm³).
Sintering Cr and Si containing PM steels requires particular attention to the sintering atmosphere since these elements form stable oxides that are difficult to reduce. The preferred sintering atmosphere is N₂-H₂ with a minimum of 5-10% hydrogen to ensure good reduction of surface oxides.
The attained ultimate tensile strength of Ancorsteel 4300 is shown in Fig. 1 depending on the sintering temperature and time. The results show clearly that a sintering time of 30 minutes is required and that the sintering temperature should be at least 1150°C, better 1180°C, to fully utilize the alloying elements. With these processing parameters the steel attains an apparent hardness of 27 HRC and the dimensional change from die exhibits a slight shrinkage of -0.11% (1150°C) or -0.18% (1180°C).
The microstructure of this material sintered at 1180°C/30 min with a cooling rate of 0.7°C/s is mostly martensite with remaining areas of bainite. The master alloy has diffused into the structure to a high degree. This set of sintering conditions gives the highest strength of the test matrix. If the cooling rate is further increased to 2.2°C/s, the apparent hardness after tempering at 205°C can be increased up to 41 HRC.
Ancorbond FLM 4400
While the grade Ancorsteel 4300 still contains a small amount of nickel in the form of a master alloy, Ancorbond FLM 4000 and Ancorbond FLM 4400 are completely nickel-free. The chemical composition is 1.3% Mn, 0.5% (0.8%) Mo, 0.6% graphite, balance iron. Due to the chemical reaction of carbon with oxides and the atmosphere, the final carbon content after sintering was reduced to 0.45%C.
Like chromium, manganese containing PM steels require special attention when sintering due to manganese’s higher affinity for oxygen relative to more traditional alloying elements such as nickel or copper. Dry N₂-H₂ atmospheres should be used with dew points < -40 °C with a minimum hydrogen content of 5-10 vol.%. A sintering temperature of 1120°C is sufficient to attain high strength and hardness values, but high temperature sintering at 1260°C leads to less dimensional growth and complete homogenisation of the master alloy.
The Mn alloy steels exhibit a significant growth after sintering. At 1120°C the dimensional change is +0.4% and at 1260°C it is +0.3%. Slight differences may occur depending on the cooling rate.
The most pronounced effect on the mechanical properties is that of the cooling rate, as shown in Fig. 3. All samples were tempered at 205°C/1 h after sintering. The strength depends mainly on the amount of martensite formed during cooling and increases with the sintering temperature and the cooling rate. Correspondingly, the apparent hardness in-creases from around 20 HRC at 0.7°C/s to 33 HRC at 2.2°C/s.
While atmosphere heat-treatment and quenching is difficult with this alloy system it has been shown that this material system can very well be locally induction hardened because the time at the temperatures where oxidation takes place is very short.
Microstructures of the test samples were also inspected. An example is shown in Fig. 4. The sample cooled at 1.6 °C/s has a microstructure that is mostly martensite with a small area fraction of bainite. This hardened structure explains the high strength and hardness observed. The high temperature sintered samples also have a microstructure that is mostly martensite, but the porosity is smaller and more rounded.
The master alloy concept is attractive for high strength PM parts that use sinter-hardening. It avoids the use of free nickel and offers relatively low alloying costs for a good response to heat treatment.
Author: Dr Georg Schlieper, Gammatec Engineering GmbH, Germany
Dr.-Ing. Georg Schlieper, physicist, received his PhD at the Insitute for Materials and Solid State Research of the University of Karlsruhe, Germany. He worked for 15 years in product and process development for the Powder Metallurgy industry where he focused on high strength sintered steels, heat treatment, surface technology, magnetic materials and metal injection moulding. Since 1994 he has worked independently as a consultant. Email: [email protected]