EFFECT OF POST SINTERING THERMAL TREATMENTS ON DIMENSIONAL PRECISION AND MECHANICAL PROPERTIES IN SINTER-HARDENING PM STEELS

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1 EFFECT OF POST SINTERING THERMAL TREATMENTS ON DIMENSIONAL PRECISION AND MECHANICAL PROPERTIES IN SINTER-HARDENING PM STEELS Bruce Lindsley and Thomas Murphy Hoeganaes Corporation Cinnaminson, NJ USA ABSTRACT Dimensional precision is a critical parameter in net shape processing of ferrous PM components. PM parts producers continue to pursue larger parts, but absolute tolerances dictated by the end user generally do not scale with part size. Therefore, in larger parts, the variation in percentage change in size, or dimensional change, must be reduced. Beyond the dimensional changes associated with pressing and sintering of typical low alloy PM steels, sinter-hardenable alloys present some unique challenges and opportunities for PM part manufacturing. The ability to harden a part in the sintering furnace eliminates the need for a secondary quenching operation. The resulting microstructure of untempered martensite is, however, not ideal for dimensional stability and mechanical properties. Tempering hardened steels results in improved mechanical properties and dimensional shrinkage, as the martensite converts to a more stable ferrite and carbide microstructure of higher density. In addition, many sinter-hardening grades contain high Cu and C contents that result in relatively high amounts of retained austenite. Retained austenite can improve impact and ductility properties, but contributes to dimensional instability as it can transform to lower density bainite and/or martensite with thermal fluctuations. Proper thermal treatments of sinter-hardened steels are necessary to obtain the best combination of mechanical properties and dimensional control. This paper reviews the effects of different post-sintering thermal treatments on the dimensional, microstructural and mechanical property changes of sinter-hardened PM steels. INTRODUCTION Dimensional change in ferrous sintered compacts is influenced by several factors, including particle size, density, composition (alloying method), sintering time and temperature, cooling rate and microstructure [1-6]. Given a base iron and sintering conditions, factors such as admixed composition and cooling rate play an important role on microstructure, dimensional change and mechanical properties. Previous studies detail the effects of composition on dimensional change of sinter-hardenable steels [1,5]. Upon cooling from the sintering temperature, the high temperature phase in steel, austenite, transforms to either

2 ferrite plus carbide (in the form of lamellar pearlite, divorced pearlite or bainite), martensite, or a combination thereof. The type of martensite formed is dictated by the carbon content. At carbon contents less than 0.6%, lath martensite forms. Above 0.8%, acicular (plate) martensite forms, and a combination of both is found in alloys between 0.6 and 0.8%. The volume change of a martensite type is also controlled by the carbon content. Sinter-hardening utilizes alloyed steels and/or accelerated cooling rates to achieve fully hardened, martensitic structures directly out of the sintering furnace. In addition, austenite is often present in the microstructure in heavily alloyed regions (retained austenite). Austenite is the highest density phase in steel, followed by ferrite plus carbide, and finally martensite, which is the lowest density phase. This decrease in density results in a corresponding increase in compact size, and martensite formation leads to the largest growth resulting from microstructural changes. Length changes in pore free materials can reach 1.4% upon transformation from austenite to martensite [7]. Tempering of martensite leads to a ferrite and carbide structure, which has a higher density. The post sintering operation of tempering should therefore decrease the dimensional change of a martensitic sintered compact. However, retained austenite is not a stable phase at temperatures below the eutectoid temperature. A reduction in temperature below room temperature will continue to drive the martensite transformation, while an increase above ambient temperature will encourage bainite formation. Transformation of retained austenite to either martensite or bainite will result in compact growth. The purpose of this paper is to investigate the effects of post-sinter processing on the dimensional change and microstructure of sinterhardened steels. EXPERIMENTAL PROCEDURE The alloys studied were FLC (Ancorsteel wt% Cu + graphite), FLNC-4408 (Ancorsteel 85HP + 2wt% Ni + 1.5wt% Cu + graphite), FLC-4608 (Ancorsteel wt% Cu + graphite) and Ancorsteel graphite. The compositions are listed below in Table I. The mixes were compacted into standard TRS, dogbone tensile, Charpy impact and dilatometry test specimens at a compaction pressure of 690 MPa (50 tsi) at room temperature. The samples were sintered in a 90% nitrogen 10% hydrogen atmosphere at 1120 C and accelerated cooling was used to achieve a cooling rate of 1.6 C/sec (2.8 F/sec) between part temperatures of 650 C (1200 F) and 315 C (600 F). Tempering was carried out in a nitrogen purged furnace at 205 C (400 F) for the majority of test conditions. After sintering, samples were subjected to a variety of tempering (T) and liquid nitrogen quench () operations: 1. assintered, 2., 3. T/1hr, 4. + T/1hr, 5. T/1hr, cool to room temperature + T/1hr, 6. T/1hr +, 7. T/1hr + T/1hr +, 8. T/4hr, 9. T at 225 C/1hr (435 F/1hr). Dimensional change (DC) from die size and mechanical properties were evaluated for all nine conditions. Table I. Nominal compositions (in wt.%) of the alloys studied, balance Fe. Alloy # Designation Ni Mo Mn Cu Cr Si C 1 FLC FLNC FLC Ancorsteel Ancorsteel is a registered trademark of Hoeganaes Corporation

3 RESULTS The as-sintered microstructure of the four sinter-hardenable alloys is shown in Figure 1. Alloy 1 (FLC2-4808) is fully martensitic, while the other three alloys are predominately martensitic with a small percentage of bainitic regions. Nickel-rich regions can be found in the FLNC-4408 (Alloy 2) microstructure. Alloy 1 has the highest hardenability of the four alloys and can be cooled at a relatively slow rate while still maintaining a fully martensitic microstructure. The other three alloys have lower hardenability, and cooling rate of the sintered compact will play an important role in the final microstructure. One should be aware that the amount of martensite will play a role in the final dimensions of the part and how it responds to tempering. Therefore cooling rate must be well controlled to develop reproducible microstructures and dimensions. Alloy 1 : FLC Alloy 2 : FLNC-4408 Alloy 3 : FLC-4608 Alloy 4 : Ancorsteel 4300 Figure 1. As-sintered microstructures of sinter-hardened Alloys 1-4 (2% Nital/4%Picral Etch).

4 Alloys high in carbon and copper with martensitic microstructures, such as Alloys 1-3, often contain a large fraction of retained austenite. Figure 2a shows the martensitic microstructure of Alloy 1 using a typical etchant, whereas Figure 2b shows the same field etched to reveal retained austenite (white). The majority of the retained austenite is directly associated with copper rich regions. Notice the retained austenite is present surrounding the pore network where copper concentrations are highest. Earlier work on this alloy has shown retained austenite levels as high as 10% [1]. Given that the retained austenite is not uniformly distributed, local concentrations of austenite near the pore network will be much higher than the average value reported in the earlier work. Dimensional control of these sinter-hardened structures will be governed by not only tempering of martensite but also the transformation of retained austenite. (a) 2% Nital / 4% Picral (b) as (a), then Na 2 S 2 O 5 in Water Figure 2. Microstructure of the sinter-hardened Alloy 1 (FLC2-4808) at 1.6 C/sec etched to reveal (a) martensite and (b) retained austenite (white) in Cu-rich regions (same field). The early stages of tempering involve 3 phenomena: 1) rapid stress-relieve and diffusion of C to dislocations at relatively low temperature, 2) transformation of retained austenite to bainite, and 3) further C diffusion out of the martensite associated with the formation of carbides above 200 C (400 F) [7]. The first and third phenomena involve a reduction in length, as the body center tetragonal (bct) structure of martensite changes to bcc ferrite and carbide. The second produces a length increase, as the high density austenite phase transforms to the lower density bainite. To demonstrate these three stages of tempering, dilatometry was run on sinter-hardened Alloy 1. Figure 3 shows the change in length with temperature and it can be seen that the slope of line upon heating is not constant. Between 125 and 175 C (260 and 350 F), the slope is greatly reduced as a result of the first stage of tempering. This rapid change does not require a long time at temperature, as it occurs during heating prior to reaching the final temperature. At the 1 hour hold temperature of 205 C, a small amount of growth can be seen (see inset box) due to retained austenite transformation. Upon cooling to room temperature, the reduction in sample length is observed (-0.09%). To better illustrate the tempering effects at temperature, two samples were heated to 225 C (435 F) to accelerate the dimensional changes. The first sample was quenched in liquid nitrogen prior to tempering in order to transform retained austenite to martensite. A greater length decrease (-0.14%) is observed as the sample is heated and held at temperature (Figure 4a). This length decrease during the hold is a result of the third stage of tempering. No growth is observed as the retained austenite was removed prior to the dilatometric test. The sample that was not quenched in

5 liquid nitrogen (Figure 4b) shows a different behavior. Growth is observed during the one hour hold at temperature. This growth is a combination of shrinkage as the martensite is tempered and a larger positive growth as the retained austenite is transformed to bainite. The growth at 225 C is greater than that at 205 C, indicating that more retained austenite transforms at the higher temperature. Overall, the sample length decreased by only -0.07%. By understanding these phenomena, the effects of post sintering treatments on dimensional change and stability can be better understood in PM parts. Dilation ( µm ) Heating Cooling Temperature ( o C ) Figure 3. Tempering of Alloy 1 (FLC2-4808) in the dilatometer at 205 C (400 F) for 1 hour. Dilation ( µm ) Heating Cooling Temperature ( o C ) Dilation ( µm ) Heating Cooling Temperature ( o C ) a b Figure 4. Tempering of Alloy 1 at 225 C (435 F) a) after a liquid nitrogen quench and b) as-sintered. The dimensional change of Alloys 1-4 for the various thermal treatments is given in Table II and Figure 5. The results have been normalized to an as-sintered dimensional change of zero. Alloys 1 3 have significant levels of retained austenite, resulting in significant growth (+0.13% to +0.14%) with the. Tempering of the martensite results in shrinkage, as the bct structure of martensite converts to a bcc ferrite and carbide structure. Shrinkage results are seen in Table II, where the pre and post temper

6 condition of the as-sintered and samples are compared. More shrinkage occurs during tempering of the sample, as there is no growth contribution from conversion of retained austenite. It is interesting to note that the growth of Alloys 1-3 upon was similar, but the shrinkage during tempering is consistently lower for Alloy 2. The double tempered samples and the samples tempered for 4 hours (conditions 5 and 8, respectively) show growth relative to the single tempered samples as the retained austenite transforms to bainite. The growth of samples that undergo is greatly reduced if they have been tempered previously, conditions 6 & 7. Samples only grew between 0.05% and 0.08% relative to the tempered condition, whereas the samples grew between +0.13% and +0.14%. This is consistent with a portion of the retained austenite transforming during the temper, and thereby less is present to transform to martensite during the. The T + T + samples have nominally the same DC as the T + samples. Given that the T + T operation resulted in more growth relative to the single T operation and thereby had less retained austenite after tempering, it is expected that the T + T + will have more bainite and less martensite than the T +. The 225 C temper for 1 hour resulted in both tempering of the martensite and transformation of the austenite, and is expected to be the most dimensionally stable sample of the single step operations. All of the results are consistent with the tempering stages developed for wrought alloys. Finally, the tempering difference between alloys 1 & 3 and alloy 2 is likely a result of the admixed Ni in Alloy 2. The retained austenite and martensite in the Ni-rich regions is expected to behave differently from the rest of the microstructure. Table II. Dimensional change (%) of post sinter operations normalized to the as-sintered length. Alloy 1 is FLC2-4808, Alloy 2 is FLNC-4408, Alloy 3 is FLC-4608 and Alloy 4 is Ancorsteel 4300 with 0.6% graphite. Alloy # 1. As- Sinter T 4. + T 5. T + T 6. T + 7. T + T + 8. T/4hr 9. T/ 225 C The dimensional change results indicate that higher carbon alloys 1-3 have greater amounts of retained austenite and highly stressed plate or acicular martensite, due to the large growth resulting from the and the large shrinkage due to the tempering, respectively. Alloy 4 is relatively insensitive to post sintering thermal treatments. This is a result of the lower carbon content (0.53% sintered C) and no admixed copper. The improved hardenability of Alloy 4 via a combination of Cr, Ni, Mo and Si allows this lower carbon content to be used in sinter-hardening. The lower carbon content results in lath martensite formation, which is less stressed than the higher carbon acicular martensite. Shrinkage upon tempering is therefore reduced. The lower carbon and lack of copper greatly reduce the amount of retained austenite, thereby reducing the sintered compact growth upon. Alloy 4 had a much smaller growth (+0.06%) resulting from the quench as compared to Alloys 1-3 that had growths more than double this amount. Alloy 4 is therefore more dimensionally stable after sintering.

7 0.15 FLC FLNC FLC-4608 Dimensional Change (%) T + T T + T T + T +T + Ancorsteel 4300 T / 4hr T / 225C Figure 5. Dimensional change from as-sintered size due to different post sintering thermal treatments. Given that mechanical properties are governed by the structure of the part, it is reasonable to assume that the mechancial properties would follow these changes in dimensions. Table III shows the mechanical properties of Alloy 1 in the 9 different thermal treatment conditions. The as-sintered hardness is quite high in this alloy, while the other as-sintered properties are relatively low. As a reference, 60 HRA is equivalent to 20 HRC and 70 HRA is equivalent to 39 HRC. The highly brittle martensite caused the tensile samples to fail prior to yielding, resulting in extremely low tensile strengths in this alloy. One property that provides reasonable performance in the as-sintered condition is impact energy, which is aided by the high amount of tough retained austenite. causes all of the mechanical properties to decrease, except for hardness. The increase in hardness is consistent with retained austenite transformation to martensite. Tempering of this alloy results in a dramatic improvement in properties. While the stress-relief during tempering reduces hardness, TRS doubles and tensile strengths and elongations more than double. It is interesting to note that once the martensitic structure has been tempered, little change in mechancial properties occurs with further post-sintering thermal treatments. Continued tempering of martensite or transformation of retained austenite by either or prolonged, elevated or repeated tempering has little effect on mechanical properties. It appears that the stress-relief that was observed in the dilatometer between 125 C and 175 C is the dominate stage of tempering for the development of good mechanical properties. The results also suggest that tempering conditions can be modified to fine tune the dimensions of a part with little effect on mechanical properties.

8 Table III. Mechanical properties of Alloy 1 with different post sintering thermal treatments. Sinter Condition Density Hardness TRS 0.2% YS UTS Elong. Impact (g/cm³) (HRA) (MPa) (MPa) (MPa) (%) (J) As-sintered T T T + T T T + T T for 4 hrs T at 225 C The other 3 alloys were also mechancially tested in all 9 conditions. The hardness and TRS for the four alloys is displayed in Table IV and Figure 6. Alloy 2 and 3 followed similar trends as Alloy 1. The assintered and the samples had low strengths, while tempering essentially doubled TRS and increased tensile and impact properties. Again, further thermal treatments after a single temper had little effect. Alloy 1 had the best combination of hardness and strength for the first 3 alloys. Table IV. Hardness and TRS for all four alloys with the different thermal treatments. Alloy 1 - FLC Alloy 2 - FLNC-4408 Alloy 3 - FLC-4608 Alloy Condition Hardness TRS Hardness TRS Hardness TRS Hardness TRS (HRA) (MPa) (HRA) (MPa) (HRA) (MPa) (HRA) (MPa) As-sintered T T T + T T T + T T for 4 hrs T at 225 C

9 Alloy 4 exhibited a different behavior. The as-sintered properties were the best of all alloys tested and the had little effect on properties. Tempering improved TRS and tensile properties by roughly 25%, and while significant for developing the best properties, tempering was much less important in this alloy system compared to Alloys 1-3. Again, this is a function of the martensite type that is present in the lower carbon steel. As-sintered hardness was lower than Alloys 1 and 3, but after tempering, Alloy 4 had similar hardness. The transverse rupture strength of Alloy 4 is equivalent or better than Alloy 1 (Table IV) and the tensile properties are similar. It should be noted that the sintered density of Alloy 4 was 0.05 to 0.10 g/cm 3 higher than the other alloys due to a combination of improved compressibility and less growth during sintering. Given the ability to properly sinter a Cr-containing PM alloy, Alloy 4 provides a more dimensionally stable sinter-hardening system as compared with more traditional sinterhardening alloys TRS (MPa) Alloy 1 Alloy 2 Alloy 3 Alloy As-sintered T + T T + T T + T + T + Figure 6. TRS strength of the 4 alloys with different thermal treatments. T for 4 hrs T at 225 C SUMMARY The dimensional and microstructural changes of different sinter-hardening grades were investigated with different post-sintering thermal treatments. Tempering of martensite reduces the growth associated with its formation and produces a less brittle structure. Tempering is critical to mechanical properties in sinter-hardened steels. It also improves the dimensional stability of the material. The presence of retained austenite in high carbon alloys can lead to dimensional instability of a PM component. If cooled to low temperature, the austenite will transform to untempered martensite and result in large growth. If heated to a high enough tempering temperature, the austenite will transform to bainite. The lower carbon martensite produced in Alloy 4 (Ancorsteel 4300) resulted in little dimensional change during the various

10 post sintering thermal treatments and provided equivalent or better mechanical properties than the more traditional sinter-hardening alloy systems. The most dimensionally stable structure will contain little to no retained austenite and well tempered martensite. REFERENCES 1. B. Lindsley, G. Fillari and T. Murphy, Effect of composition and cooling rate on physical properties and microstructure of prealloyed P/M steels, Advances in Powder Metallurgy & Particulate Materials, compiled by C. Ruas and T. A. Tomlin, Metal Powder Industries Federation, Princeton, NJ, 2005, part 10, p N. Dautzenberg and H. J. Dorweiler, Dimensional behavior of copper-carbon sintered steels, Powder Metallurgy International, Vol. 17, No. 6, 1985, p R. L. Lawcock and T. J. Davies, Effect of carbon on dimensional and microstructural characteristics of Fe-Cu compacts during sintering, Powder Metallurgy, Vol. 33, No. 2, 1990, p F. Chagnon and M. Gagne, Dimensional control of sinter hardened P/M components, Advances in Powder Metallurgy & Particulate Materials, compiled by W. G. Eisen and S. Kassam, Metal Powder Industries Federation, Princeton, NJ, 2001, part 5, p B. Lindsley and T. Murphy, Dimensional Control in Cu-Ni Containing Ferrous PM Alloys, Advances in Powder Metallurgy & Particulate Materials, compiled by W. R. Gasbarre and J. W. von Arx, Metal Powder Industries Federation, Princeton, NJ, 2006, part 10, p T. Singh and T. F. Stephenson, Ni-Cu-Mo interactions in sinter-hardening steels, Euro PM2005, EPMA, vol. 1, p R. E. Reed-Hill, Physical Metallurgy Principles 2nd Edition, PWS-Kent Publishing, Boston, MA, 1973, p