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Manuscript refereed by Chairman Name (Company, Country)

High Performance Powder Metallurgy (PM) Stainless Steel Solutions Roland T. Warzel III (North American Höganäs, 111 Höganäs Way, Hollsopple PA 15935, USA) roland. warzel@nah.com; Bo Hu (North American Höganäs, 111 Höganäs Way, Hollsopple PA 15935, USA) bo.hu@nah.com; Abstract Numerous applications utilize components manufactured from powder metallurgy (PM) stainless steels. While PM stainless steel components meet very demanding performance criteria, the overall PM stainless steel market is quite small compared to ferrous PM. Stainless steel powders often have poor powder properties which makes the manufacturing of stainless steel PM components challenging, often leading to lower productivity to meet quality goals. The components often require secondary machining in order to meet final dimensions or add features which couldn’t be achieved through PM compaction and sintering processes. The machining of PM stainless steel can be challenging due to the inherent microstructure of stainless steel and the resulting low particle hardness of matrix. Recent material advances in PM stainless steels now offer solutions to these common problems. This paper will discuss the recent advances detailing the improvements in powder properties and machinability which can be expected for common PM stainless steels.

Introduction Although a number of different applications are currently in production, the overall market for stainless steel PM components is rather small, especially compared to the ferrous market. While a number of factors play a role in the small market size, the overall manufacturability of the powders is a significant contributor.Stainless steel powders are produced through water atomization resulting in powder particles which are irregular in shape, apparent densities in the range of 2.8 – 3.4 g/cm3 and fine fractions (-45 µm) between 35 – 50% [1]. The irregular shape and large fine fraction result in powders which have poor flow and green strength compared to ferrous iron powders [2]. The two most common lubricants which are utilized in stainless steel mixes currently are ethylene bisstearamide (EBS) and lithium stearate. Each lubricant has positive and negative effects on the manufacturability when used with stainless steel powders. EBS typically results in high green strengths and low apparent densities, while lithium stearate results in high apparent densities but low green strengths. The flow aspect of both lubricants are typically poor. Stainless steel components undergo shrinkage during the sintering process as the particles bond and sinter together. The higher the temperature, the more shrinkage occurs. In the case of ferritic stainless steel components, it is not uncommon to achieved 2-4% shrinkage after sintering. This amount of shrinkage can make holding tight tolerances a challenge and often result in a secondary machining step. Other times, machining can be utilized to add features not possible during compaction or to provide a certain surface finish [3]. Due to the soft microstructure of ferritic and austenitic stainless steel alloys, machining can be difficult due to their tendency to become gummy or work harden during the machining process. In order to overcome the challenges related to the powder properties and machinability of stainless steel materials, development activities have resulted in an advanced lubricant system specifically designed for stainless steel powders and a unique grades of machinable stainless steel powders. This paper will detail the performance of these recent developments. Experimental Procedure Lubricant Comparison Study

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Commercially available austenitic and ferritic grades of stainless steel powder were chosen for the lubricant comparison. All of the grades were nominally -150 µm in particle size. The details of the base powder grades are shown in Table 1.

Table 1. Chemical composition & particle size of base powders Grade Cr (%) Ni (%) Mo (%) S (%) C (%) -45 µm (%) 304L 18.4 11.0 - 0.01 0.021 45 316L 16.8 13.4 2.2 0.01 0.021 45

410L 12.1 - - 0.01 0.017 47 430L 16.9 - - 0..01 0.020 47

All grades were mixed with 1 wt% of the lubricants EBS, lithium stearate and Intralube® F in a standard mixer. After mixing, samples were obtained in accordance with industry standards [4]. The powder mixes were tested for apparent density and flow rate. Compaction curves were generated for the mixes on 25 mm diameter slugs at compaction pressures of 400, 600 and 800 MPa. Mechanical testing was performed on transverse rupture strength bars compacted to a green density of 6.5 g/cm3. Sintering was conducted at 1290 °C in a 100% hydrogen atmosphere for 30 minutes. The test bars were measured for hardness, dimensional change and transverse rupture strength. All testing was conducted in accordance with MPIF standards [5]. Machinability Testing - Laboratory The performance of 304L, 316L, and 430L and their Stainless Steel EZTM counterparts have been detailed in past studies. [6-7]. For this study, the alloys 303L and 410L were evaluated against their machinable versions. The chemical composition of the alloys are shown in Table 2.

Table 2. Chemical composition & particle size of the base powders Grade Cr (%) Ni (%) S (%) C (%) -45 µm (%) 303L 18.2 12.6 0.20 0.021 45 303L-EZ 18.2 12.6 0.20 0.020 45 410L 12.7 - - 0.018 47 410L-EZ 12.7 - - 0.018 47

The base powders were lubricated with 1 wt% of EBS and compacted into ɸ55 mm x ɸ35 mm x H20 mm ring specimens at a green density of 6.5 g/cm3. The rings were then sintered in a commercial furnace 1290 °C in a 100% hydrogen atmosphere for 30 minutes. The ferritic materials were evaluated for machinability by turning the inner diameter of the rings. The austenitic materials were evaluated for machinability by turning the inner diameter and by facing the surface. All machining was conducted using a NC machine at the North American Höganäs Pilot Machining Center. TiCN coated inserts manufactured by Iscar was used for the testing. The machining was conducted in the wet condition using a water based cutting fluid and consistent cutting speed. The parameters for machining are listed in Table 3.

Table 3. Machining Parameters Parameter ID Turning Facing

Cutting Speed 550 smm 550 smm Feed Rate 0.1 mm/rev 0.1 mm/rev Depth of Cut 0.5 mm 0.5 mm Length of Cut 20 mm 20 mm

Machinability was evaluated by evaluating the tool wear after a certain machining distance using a Hitachi S-2600N scanning electron microscope (SEM). The insert then resumed machining. This procedure was continued until the testing was terminated.

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Results – Lubricant Comparison The powder property results for the mixes are shown in Table 4. The apparent density and flow rate of the mixes which utilized the new lubricant system were vastly improved compared to the industry standard lubricant. Apparent densities similar to lithium stearate were achieved with very good flow rates. The improvement in green strength was significant for the new lubricant system.

Table 4. Powder Property Results

Material Apparent Density

g/cm3 Flow Rate

s/50g Green Strength

MPa

304L + 1% EBS 2.66 37.7 7.7 304L + 1% LiSt 2.88 38.1 5.8 304L + 1% Intralube F 2.88 30.0 11.0

316L + 1% EBS 2.66 37.8 8.2 316L + 1% LiSt 2.90 35.5 4.8 316L + 1% Intralube F 2.84 31.9 13.8 410L + 1% EBS 2.74 36.8 10.4 410L + 1% LiSt 2.98 37.0 8.3 410L + 1% Intralube F 2.97 28.7 14,5

430L + 1% EBS 2.77 35.1 8.1 430L + 1% LiSt 2.98 35.6 7.0 430L + 1% Intralube F 3.00 28.9 11.1

The compressibility curves for the mixes are shown in Figure 1.

Figure 1. Compressibility Curves

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The compressibility of the LiSt and Intralube F mixes were similar. The EBS based mixes had consistently lower compressibility as expected. The results of the mechanical property evaluation are presented in Table 5.

Table 5. Mechanical Property Testing

Material Green Density

g/cm3 Sintered Density

g/cm3 Hardness

(HRB) DC (%)

TRS (MPa)

C %

304L + 1% EBS 6.5 6.81 40 -1.80 762 0.007304L + 1% LiSt 6.5 6.78 42 -1.85 758 0.040304L + 1% Intralube F 6.5 6.83 39 -1.81 766 0.008

316L + 1% EBS 6.5 6.88 36 -2.16 814 0.01 316L + 1% LiSt 6.5 6.88 36 -2.23 758 0.02 316L + 1% Intralube F 6.5 6.91 36 -1.9 807 0.01

410L + 1% EBS 6.5 7.30 61 -4.24 1076 0.005410L + 1% LiSt 6.5 7.35 60 -4.44 1067 0.009410L + 1% Intralube F 6.5 7.32 62 -4.40 1088 0.004 430L + 1% EBS 6.5 7.08 62 -3.08 1235 0.004430L + 1% LiSt 6.5 7.10 65 -3.04 1243 0.007430L + 1% Intralube F 6.5 7.11 60 -3.11 1241 0.003

Similar values were found for all of the mechanical properties tested. The lubricant systems did not have an appreciate effect on the mechanical properties. Results – Machinability The results of the machinability evaluation in the laboratory for the 410L & 410L-EZ materials are shown in Figure 2.

Figure 2. Machinability comparison between 410L and 410L-EZ

At both machining conditions, the 410L-EZ material exhibited a large improvement in machinability. After an initial wear, the 410L-EZ sustained a consistent amount of wear over the distance cut. This is in contrast to the 410L material which had increase wear until the tool failed. There was significant wear on the inserts which machined the base material. Only a small amount of flank wear was observed on the inserts which machined the 410L-EZ. Similar performance was found for the 303L material. The results are shown in Figure 3.

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Part ID 521 522

material 303L 303L-EZ

Facing

no. of cuts 440 440

tool wear 305um 122um

tool status broken ok

ID turning

no. of cuts 200 200

tool wear 220um 103um

tool wear worn ok Figure 3. Machinability summary for 303L and 303L-EZ (top row: turning, bottom: facing)

The 303L material was significantly more difficult to machine. The base 303L-EZ material had a small amount of wear under both machining operations. Discussion Current industry practice in developing mixes using stainless steel powders is to utilize the standard lubricants of EBS and lithium stearate. In some cases, the two lubricants are used together in order to find a compromise of performance. In developing a lubricant specifically designed for the unique powder characteristics of stainless steel powders, exceptional performance for apparent density and flow rate can be achieved. By improving the powder properties, the new lubricant should allow for faster fill shoe speeds, increasing productivity at the compaction press. Another unique feature of the new lubricant is the improvement in green strength. This allows for a more robust compaction process by reducing the likely hood of cracks due to handling. It also provides the opportunity to produce components with intricate features or severe density gradients due to the higher green strength. The machinability of sintered materials was evaluated in the laboratory scale and confirmed the poor machinability of standard grades of stainless steel. Using the machinable versions of the grades, a large improvement in machinability was achieved. This is due to the modified nature of the machinable stainless steel powders providing the opportunity for material to chip and break away from the surface instead of adhering to the insert. This function allows the insert to maintain a sharp cutting edge, increasing the overall insert life. Additional analysis was conducted on the 303L materials. Surface finish was evaluated to evaluate the potential differences between the two materials. The surface photographs are shown in Figure 4.

Figure 4. Surfaces of 303L and 303L-EZ specimens after ID turning

A large difference in surface appearance was observed. The 303L-EZ specimens had half of the surface roughness compared to the standard material. This confirms the tool wear measurements in the laboratory study. The machinable grades help with chip breaking and prevention of material

World PM2016 – Session Name 

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buildup on the insert. This results in a tool piece which stays sharp and provides consistent cutting performance. The same observation was found in another study. The production trial indicated that the machinable grade provide superior performance during machining resulting in very good surface finish. The material removal was much better compared to the standard material [8]. Conclusions Based on this study, the following conclusions can be drawn:

1. Improved apparent density, flow rate, and green strength were achieved using the Intralube F lubricant system. Compared to EBS and lithium stearate, an improvement in flow rate of 5 s/50g was achieved with the new lubricant. The green strength improvement was 70% over EBS which is regarded as the best lubricant for green strength in stainless steel mixes

2. Improvement machinability was observed when using the 303L-EZ and 410L-EZ base powders. Both materials were able to machined significant distances with only a small amount of wear on the insert.

3. An improvement in surface finish was observe for the machinable grades. The improved chip breaking properties of the 303L-EZ material allowed for the insert to maintain cutting ability and provide a smooth surface finish.

Acknowledgements The authors would like to acknowledge Amber Neilan and Sarah Ropar of North American Höganäs for their assistance in manufacture and testing of the powders and specimens. References

1. E. Klar, P. Samal: Powder Metallurgy Stainless Steels, ASM International, (2007), pg 25-27.

2. R. Warzel III, “Stainless Steel Mixes with Improved Performance”, Advances in Powder Metallurgy & Particulate Materials compiled by R.A. Chernakoff and W.B. James, Metal Powder Industries Federation, Princeton, NJ, 2014, part 3, pp 49 -61.

3. R. German: Powder Metallurgy & Particulate Materials Processing, Metal Powders Industries Federation, p. 338

4. ASTM International Standard B 215-15, Standard Practices for Sampling Metal Powders. 5. Standard Test Methods for Metal Powders and Powder Metallurgy Products, 2016 edition, Metal

Powder Industries Federation, Princeton, NJ. 6. B. Hu, R. Warzel, P. Samal, S. Luk, “Development of Easy Machinable Stainless Steel Powder for

Manufacturing Sintered Stainless Components”, Advances in Powder Metallurgy, complied by I. Donaldson and N.T. Mares, Metal Powder Industries Federation, Princeton, NJ, 2012, part 7, pp. 88-99.

7. B. Hu, R. Warzel, S. Luk, “Machinability Enhancement of PM Stainless Steels using Easy Machinable Stainless Steel Powder”, Advances in Powder Metallurgy, complied by D. Christopherson and R.M Gasior, Metal Powder Industries Federation, Princeton, NJ, 2013, part 7, pp. 73-86.

8. B. Hu, R. Warzel, R. Neyman, P. Samal, “Manufacturing 400 series components using Easily Machinable Stainless Steel Powders, International Journal of Powder Metallurgy 2016