Heat Treatment and Sintering Effects on High Density Powder Metallurgical 4600 Series/AISI 9310

Low-Alloy Steels and Select Wrought Steels

Karthik Nagarathnam Senior Materials Scientist

and Dennis Massey CEO/President

Utron Kinetics, LLC 9441 Innovation Dr. Manassas, VA 20110

Technical Paper & Presentation at the 2009 PM Conference, Los Vegas, NV, June 28-July 1-2009.

Abstract Low-alloy steel materials and ferrous alloys/composites continue to find several applications in the rotorcraft/power transmission components in the PM industry. Higher density and high performance PM low alloy steel parts using select alloy compositions have been fabricated using high pressure Combustion Driven Compaction (CDC) process.[10-16] The CDC process was used to compact variety of low-alloys steels of 9310 equivalent low-alloy steel mix compositions to get mechanical properties better than 4600 series alone as well as using commercially available and highly compressible 4600 series-Base powders relative to gas-atomized AISI 9310 steel powders of -325 and -635 mesh sizes at 150 tsi and post-process thermal cycles were carried out to evaluate their CDC compaction responses and materials behavior. In this project, we are focusing on the sintering/heat treatment responses on the microstructure, geometrical, physical and room temperature mechanical properties. For the CDC samples, sintering is done first and then followed by heat treatments and for wrought AISI 9310 steel/ AISI 4340 steels, only the heat treatments were done of as-received material. Several CDC high pressure compacted alloy mixes were processed using different combinations of sintering/heat treatment profiles in the temperature of up to 1300 °C (2370 °F) in select environments. Materials properties such as microstructure and mechanical properties were evaluated as a function of different thermal processing cycles. The basis for this study is focused on optimizing the thermal processing to obtain the best mechanical properties and establish protocols for further process optimization. Studies resulted in an understanding of the CDC compacted PM low-alloy powder mixes as they relate to their microstructures and mechanical properties, depending on thermal post-process conditions. Introduction Conventional processing of wrought low-alloy steels by processing such as hot-rolling, extrusion, forging, machining and heat treatment techniques as well as by low pressure (<760 MPa (<55 tsi)) powder compaction techniques followed by suitable sintering and heat treat cycles and the material properties attainable are well-known in the power transmission applications as evident from the published literature.[1-9] Recent advances in advanced high pressure PM compaction such as Combustion Driven Compaction [10-16] processing provide an opportunity for improved densification, key mechanical property enhancement such as higher ductilities/toughness and performance, under key process optimization strategies of high strength low-alloyed steels in critical military and civilian commercial applications.

We have reported and presented some unique test results of improved surface fatigue properties when rotating contact fatigue (RCF) tested up to 2300 MPa (333, 500 psi) contact loading conditions of CDC compacted and optimally processed FLN2-4405 low-alloy steel material for potential transmission component applications such as gears with the sub-surface fracture characteristic after RCF testing similar to those normally observed only in high density or wrought counter part materials in another publication [16] and presentation at this Conference. Such results are encouraging and form the basis in this study to develop other advanced low alloy steels for higher strength applications. The research discussed here involves surface hardenable low-alloy steels of types such as 4600 series/9310 alloy systems. The use of surface hardenable AISI 9310 steels has predominantly been for rotorcraft components such as gears and bearing applications involving higher contact stress loading conditions where good wear resistance and surface fatigue properties are necessary with much of the available literature focused on the development of case-hardening gears. We have focused on developing CDC compaction technology for fabricating higher density low-alloy steel parts by select 4600 series and 9310 steel powder mix compositions as well as the heat treatment responses of the CDC compacted parts and of the wrought AISI 9310 and AISI 4340 materials. Experimental Materials and Procedures

AISI 9310 and AISI 4340 wrought steel (in the form of round bars) were also obtained for heat treatment evaluations only to establish the baseline heat treat cycle conditions for CDC compacted low-alloy steel samples. Table 1 provides the typical nominal compositions of various rotorcraft steel materials.

Table 1. Typical Rotorcraft Materials Compositions [1, 2, 9]

Combustion Driven Compaction (CDC) and Thermal Processing [10-16]

The basic principle of the CDC process using controlled combustion of methane and air is shown in Figure 1a. A 300 ton CDC manual press (Figure. 1b) was used for all the compaction at 2100 MPa (150 tsi). A typical CDC loading cycle is shown in Figure 1c which indicate the higher compaction pressures and faster (milliseconds) process cycle time.

Alloy C S O N P Si Mn Cu Ni Mo Cr V Al Co737 SH 0.004 0.008 0.120 0.002 0.008 0.001 0.43 0.10 1.40 1.26 0.031000C 0.003 0.009 0.060 <0.001 0.007 0.003 0.11 0.06 0.04 0.02 0.04FC-0205 0.002 0.007 0.142 0.013 0.010 0.20 2.00Astaloy CrL 0.20 1.50Astaloy A <0.01 0.110 0.20 1.90 0.55

Pyrowear 53 0.100 1.000 0.35 2.00 2.00 3.25 1.00 0.10CBS-600 VIM-VAR 0.190 1.100 0.60 1.00 1.45 0.06

AISI E9310 0.08-0.13 ≤0.025 ≤0.025 0.280 0.55 3.25 0.12 1.20AISI E9310H 0.07-0.13 ≤0.050 0.230 0.55 3.25 0.12 1.23

AISI 8620 0.18-0.23 ≤0.04 ≤0.035 0.15-0.35 0.7-0.9 0.4-0.7 0.15-0.25 0.4-0.6AISI 8740 0.38-0.43 ≤0.04 ≤0.035 0.230 0.88 0.55 0.25 0.50

Pyrowear 675 0.070 0.400 0.65 2.60 1.80 13.00 0.60 5.40

P/F-42XX * ≤0.03 ≤0.03 ≤0.03 0.20-0.35 ≤0.15 0.40-0.50 0.55-0.65 ≤0.10P/F-46XX * ≤0.03 ≤0.03 ≤0.03 0.10-0.25 ≤0.15 1.75-2.00 0.50-0.60 ≤0.10

3

What is Combustion Driven Compaction (CDC)?[25-26]

+

-

Gas InletElectric

Ignition

Natural Gas (CH4)& Air at “High” Pressure

Powder Die

One moving part!

•A pressurized mixture of natural gas and air is ignited to drive a piston (ram)

•CDC converts chemical energy directly to mechanical energy for high efficiency!

(a) (b)

(c )

Figure 1 (a) CDC Basic Principle, (b) 300 Ton CDC Press (c) CDC Process Control System and d) Faster Pressing Loading Cycle Time (e.g., milliseconds)

The powder used for the CDC compaction experiments include 4600 series mix (non-spherical powder with varying size distributions) and 9310 powder (spherical powder obtained by gas atomization) equivalent mixes by blending both 4600 series base powder and gas-atomized 9310 powder at various compositions. 4600 series is a water-atomized low alloy steel [10] with Ni, Mo and Mn has been known for their good compressibility and green strength properties. Although, higher strengths have been reported for conventional PM 4600 series materials with relatively lower densities, obtaining higher densities and optimizing to obtain better mechanical ductilities (i.e., for better toughness properties) are still challenging tasks for many researchers. We have attempted to evaluate such properties. We have selected 4600 series as one of the base alloys. In general, we recognize that spherical powders are relatively difficult to compress by powder consolidation method to get the highest as-pressed densities as compared to those possible by means of powders of broad range of distributions which make them relatively easier to press by PM methods of compaction. The CDC experimental matrix involved compacting variety of other alloy compositions. (Table 2 and Table 3).

Table 2. CDC Experimental Matrix** Sample

#: Description: Green

Density (g/cm3)

Relative Density

(%)

Mass: (g)

ID/width(in)

OD/ length

(in)

Height (in)

Pore-free Density (g/cm3)

1688 4600 series+2%Ni +1.5%Cr +0.5%Mn 7.78 99.13 31.052 0.345 3.534 0.243 7.85

1689 4600 series+2%Ni +1.5%Cr +0.5%Mn 7.77 98.98 31.048 0.345 3.534 0.243 7.85

1690* 4600 series +2%Ni +1.5%Cr +0.5%Mn

(+1%Acrawax) 7.35 100.30 31.334 0.346 3.537 0.260 7.33***

1691* 4600 series +2%Ni +1.5%Cr +0.5%Mn

(+1%Acrawax) 7.31 99.85 31.345 0.346 3.538 0.261 7.33

1692 4600 series +2%Ni +1.5%Cr +0.5%Mn 7.71 98.25 160.100 0.475 1.354 1.002 7.85

1693 50% 9310(-325) / 50%(4600

series+2%Ni +1.5%Cr +0.5%Mn) 7.39 94.33 160.100 0.474 1.356 1.041 7.84

1694 75% 9310(-325) / 25%(4600 series

+2%Ni +1.5%Cr +0.5%Mn) 7.21 92.16 160.000 0.475 1.358 1.0640 7.83

1695 50% 9310(-325) / 50%(4600 series

+2%Ni +1.5%Cr +0.5%Mn) 7.55 96.40 30.549 0.346 3.53 0.246 7.84

1696 50% 9310(-325) / 50%(4600

series+2%Ni +1.5%Cr +0.5%Mn) 7.53 96.06 30.536 0.346 3.538 0.247 7.84

1697 AISI 9310 (-635) 7.83***

1698 50% 9310 (-635)/ 50%(4600 series

+2%Ni +1.5%Cr +0.5%Mn) 7.50 95.71 30.535 0.346 3.539 0.248 7.84

1699 50% 9310 (-635)/ 50%(4600 series

+2%Ni +1.5%Cr +0.5%Mn) 7.50 95.70 30.542 0.346 3.539 0.248 7.84

1700 75% 9310(-635) / 25%(4600 series

+2%Ni +1.5%Cr +0.5%Mn) 7.30 93.23 30.017 0.347 3.542 0.250 7.83

1791 AISI 9310 (-325/+635)* 7.21 92.13 30.912 0.347 0.261 7.83

1792 AISI 9310 (-635)* +1% LiSt 7.07 96.39 30.609 0.348 0.264 7.34

1793 50% 9310 (-635) / 50% 4600 series

MIX 7.41 94.61 31.077 0.346 3.540 0.255 7.84

1794 50% 9310 (-635) / 50% 4600 series

MIX 7.41 94.58 31.078 0.346 3.540 0.255 7.84

1795 4600 series +2%Ni +1.5%Cr +0.5%Mn 7.72 98.43 31.064 0.345 3.534 0.245 7.85

1796 4600 series DEMO 7.73 98.61 31.070 0.345 3.535 0.245 7.84***

1797 4600 series +2%Ni +1.5%Cr +0.5%Mn 7.73 98.58 31.048 0.346 3.535 0.244 7.85

1798 50% 9310 (-325) / 50% 4600 series

MIX 7.53 96.06 31.038 0.347 3.539 0.251 7.84

1799 50% 9310 (-325) / 50% 4600 series

MIX 7.47 95.33 31.079 0.346 3.538 0.253 7.84

DB21 4600 series+2%Ni+1.5%Cr+.5%Mn 7.73 98.58 30.990 0.346 3.533 0.243 7.85

DB22 4600 series+2%Ni+1.5%Cr+.5%Mn 7.71 98.29 30.992 0.346 3.534 0.243 7.85

DB23 4600 series MIX / 9310 (-325) 7.42 94.72 30.971 0.347 3.540 0.253 7.84

DB24 4600 series MIX / 9310 (-325) 7.48 95.53 30.959 0.347 3.539 0.250 7.84

DB25 4600 series MIX / 9310 (-635) 7.45 95.05 30.935 0.348 3.540 0.251 7.84

DB26 4600 series MIX / 9310 (-635) 7.43 94.85 30.973 0.347 3.540 0.252 7.84 *Using Admixed Acrawax Lubricant with the indicated % in the Powder; Pore-Free Density of 4600V and AISI 9310: 7.84 g/cc and 7.83 g/cc **Using no-admixed lubricant in the powder; zinc stearate diewall lubricant was used

Table 3. CDC Experimental Test Matrix**

Sample #: Description:

Green Density (g/cm3)

Relative Density

(%) Mass:

(g)

ID/ width (in)

OD/ length

(in) Height

(in)

Pore-free

Density (g/cm3)

1961 4600 series +2%Ni +1.5%Cr +0.5%Mn 7.76 98.86 31.020 0.346 3.534 0.243 7.85

1962 4600 series +2%Ni +1.5%Cr +0.5%Mn 7.74 98.63 30.980 0.346 3.533 0.244 7.85

1963 50% 4600 series MIX / 50% 9310 (-325) 7.50 95.79 30.991 0.346 3.538 0.251 7.84

1964 50% 4600 series MIX / 50% 9310 (-325) 7.49 95.58 31.017 0.346 3.538 0.252 7.84

1965 50% 4600 series MIX / 50% 9310 (-635) 7.45 95.07 31.402 0.346 3.539 0.257 7.84

1966 50% 4600 series MIX / 50% 9310 (-635) 7.43 94.84 30.991 0.346 3.540 0.254 7.84

1967 50% 4600 series MIX / 50% 9310 (-635) 7.44 94.98 30.993 0.346 3.539 0.254 7.84

R01 4600 series +2%Ni +1.5%Cr +0.5%Mn 7.74 98.65 160.130 0.475 1.354 0.999 7.85

R02 50% 4600 series MIX / 50% 9310 (-325) 7.47 95.36 160.010 0.475 1.356 1.030 7.84

R03 9310 (-325) 7.21 92.15 30.863 0.475 1.359 0.205 7.83

R04 75% 9310 (-325) / 4600 series MIX (-325)* 7.34 93.86 31.027 0.475 1.357 0.203 7.83

R05 90% 9310 (-325) / 4600 series MIX (-325)* 7.27 92.91 31.039 0.475 1.357 0.205 7.83

1968 50% 9310 (-325) / 50% 4600 series MIX (-325)* 7.52 96.00 30.969 0.347 3.538 0.251 7.84***

1969 50% 9310 (-325) / 50% 4600 series MIX (-325)* 7.51 95.88 30.959 0.346 3.538 0.251 7.84

1970 75% 9310 (-325) / 25% 4600 series MIX (-325)* 7.37 94.16 30.991 0.347 3.539 0.256 7.83

1971 75% 9310 (-325) / 25% 4600 series MIX (-325)* 7.37 94.17 30.993 0.346 3.540 0.256 7.83

1972 90% 9310 (-325) / 10% 4600 series MIX (-325)* 7.27 92.95 30.998 0.347 3.540 0.259 7.83

1973 90% 9310 (-325) / 10% 4600 series MIX (-325)* 7.26 92.75 30.983 0.347 3.540 0.260 7.83

1984 75% 9310 (-325) / 25% 4600 series MIX (-325)* 7.42 94.82 31.014 0.346 3.539 0.254 7.83

1985 75% 9310 (-325) / 25% 4600 series MIX (-325)* 7.36 94.04 30.999 0.347 3.540 0.256 7.83

1986 90% 9310 (-325) / 10% 4600 series MIX (-325)* 7.30 93.31 31.000 0.346 3.539 0.258 7.83

1987 90% 9310 (-325) / 10% 4600 series MIX (-325)* 7.23 92.44 31.006 0.346 3.540 0.261 7.83

1989 75% 9310 (-325) / 25% 4600 series MIX (-325)* 7.31 93.39 24.003 1.005 0.252 7.83

1990 75% 9310 (-325) / 25% 4600 series MIX (-325)* 7.29 93.19 24.021 1.004 0.253 7.83

1991 75% 9310 (-325) / 25% 4600 series MIX (-325)* 7.01 89.65 24.021 1.004 0.263 7.83

1992 75% 9310 (-325) / 25% 4600 series MIX (-325)* 7.04 90.02 24.027 1.004 0.262 7.83

1993 75% 9310 (-325) / 25% 4600 series MIX (-325)* 6.90 88.19 23.695 1.004 0.264 7.83

1994 75% 9310 (-325) / 25% 4600 series MIX (-325)* 6.83 87.24 24.015 1.004 0.271 7.83

1995 90% 9310 (-325) / 10% 4600 series MIX (-325)* 7.24 92.52 24.014 1.005 0.255 7.83

1996 75% 9310 (-635) / 25% 4600 series MIX (-325)* 7.22 92.32 24.010 1.006 0.255 7.83

1997 90% 9310 (-325) / 10% 4600 series MIX (-325)* 6.93 88.60 24.009 1.004 0.266 7.83

1998 75% 9310 (-635) / 25% 4600 series MIX (-325)* 6.96 88.91 24.007 1.005 0.265 7.83

1999 90% 9310 (-325) / 10% 4600 series MIX (-325)* 7.02 89.73 23.839 1.005 0.261 7.83

2000 90% 9310 (-325) / 10% 4600 series MIX (-325)* 7.17 91.69 23.868 1.005 0.255 7.83

2001 90% 9310 (-325) / 10% 4600 series MIX (-325)* 7.21 92.14 23.986 1.005 0.255 7.83

2002 90% 9310 (-325) / 10% 4600 series MIX (-325)* 6.86 87.69 24.035 1.005 0.269 7.83

2003 90% 9310 (-325) / 10% 4600 series MIX (-325)* 6.92 88.44 24.011 1.004 0.267 7.83

2004 90% 9310 (-325) / 10% 4600 series MIX (-325)* 6.69 85.45 24.024 1.004 0.276 7.83

2005 90% 9310 (-325) / 10% 4600 series MIX (-325)* 6.74 86.13 24.042 1.004 0.274 7.83

2006 9310 (-325) A1 6.85 87.61 24.595 1.006 0.275 7.83

2007 90% 9310 (-635) / 10% 4600 series MIX (-

325)(+.35%Cu) 7.19 91.77 31.041 0.345 3.544 0.263 7.84

2008 75% 9310 (-325)B1 / 25% 4600 series MIX (-

325)(+.35%Cu) 7.43 94.88 31.065 0.347 3.541 0.254 7.84

2009 75% 9310 (-325)B1 / 25% 4600 series MIX (-

325)(+.35%Cu) 7.45 95.09 31.121 0.347 3.540 0.254 7.84 **Using no-admixed lubricant in the powder; zinc stearate diewall lubricant was used; *** Pore-free densities used for Calculating Relative Density % are as follows: 9310 @7.83 g/cm3; 4600 series @7.84 g/cm3 ; 4600 series MIX @ 7.85 g/cm3 ; 4600 series MIX with 1% Acrawax @ 7.33 g/cm3; 50% 9310 (-325) / 50% 4600 series MIX (-325) @7.84 g/cm3

Vacuum Sintering of UTRON’s CDC compacted low alloy steel mixes with 4600 series compositions and mixed with 9310 alloy were carried out at 1300°C (2370°F) , followed by subsequent heat treatments. Some of the heat treatments of CDC compacted 4600 series /9310 steel alloy composite parts included normalizing (air cooling), oil-quenching and tempering cycles.

Results and Discussions

For the present tests samples of both materials went through various heat treatments and the changes in the mechanical properties were studied. All of the heat treatments listed in Table 4 were performed using a batch furnace with a nitrogen cover gas. The austenitising temperature ranged from 810°C – 1040°C (1400 °F to 1900 °F) and the hold time ranged from 30-180 minutes. Both oil and water were used as quench mediums. Tempering temperatures used were 300 °C and 600 °C (570 °F and 1110 °F) with hold times of 60-180 minutes. Macrohardness measurements were taken on all samples before and after heat treatments (Table 4). Dimensional and weight measurements were also taken to determine scale thicknesses due to heat treatments and sample density. Both the wrought as well as CDC compacted and sintered samples were heat treated (Table 4) in a batch furnace. The CDC samples were first sintered either in Nitrogen or Vacuum and then subjected to heat treat cycles. (Table 7) Samples were placed on high density alumina in the furnace. Some heat treatments were conducted using a steel chamber that was placed inside the furnace in which the compressed gas was directly ported so as to further reduce scaling of the samples during heating.

Table 4. Heat Treat Cycle Profiles

* using large bar of AISI 9310 steel The metallographic specimens were prepared by series of polishing steps followed by chemically etching with ferrous chloride etchant solution consisting of 1.0 g Fe Chloride (solid) with 2 ml HCL and 20 ml of distilled water.

Dimensional measurements on the heat treated samples indicated that there is a reduction in scaling when samples were contained in a retort with scale thickness of 12.5-37.5 micrometre compared to samples placed directly within the furnace box that varied in scale thickness between 75-262 micrometre. Table 4 provides the macrohardness properties of heat treated wrought AISI 9310 steel.

AISI 4340 Steel Behavior:

Figure 2 Microstructures of As-Rcd 4340 Figure 3 Microstructures of 4340 Wrought Steel Wrought Steel Tempered in Argon

Heat Treat

Cycle#

Austenitizing Temperature

°C (°F)

Hold Time

(minutes)

Quench Media

Hardness of As-

Received (HRC)

Hardness after

Quenching (HRC)

Tempering Temp

°C (°F)

Tempering Time

(minutes)

Retort Used

Hardness after

Tempering (HRC)

1000* 810 (1410) 30 Water 12 32.4 No 1819 950 (1740) 60 Oil 24.7 40 300 (570) 60 Yes 38.4 1516 950 (1740) 60 Water 25 14.6 600 (1110) 180 No 18.4 2000 950 (1740) 180 Oil 24.9 33.9 Yes 900* 1000 (1830) 30 Water 12 31.9 No 1700 1040 (1900) 120 Water 17 28.7 No

The as-received initial microstructure (Figure 2) of the wrought AISI 4340 steel had an average grain size of 36 µm and an average macrohardness value of ~6 HRC. The purpose of the using inert gas such as Argon gas is to minimize oxide scale formation. Select experiments at various gas flow rates, temperatures and dwell times were carried out to evaluate the responses of wrought materials. Figure 3 shows the results of microstructures and macrohardness property data for AISI 4340 steel obtained from those select experiments. There is a strong correlation between scale thickness and heat treatment temperature with an increase from 927°C to 1150°C (1700°F to 2100 °F) resulting in scale thickness increase from 152 to 698 micrometre. Increasing the Argon cover gas flow from 39 cm3/s (5 CFH) to 118 cm3/s (15 CFH) reduces the scale thickness from 698 to 250 micrometre. The macrohardness measurements do not correlate as directly with time and temperature without closely looking at the heat treatment cycles which reveal that the cooling rate for in the 1150°C / 39 cm3/s (5 CFH) experiment, average cooling rate is 7 °C/min (45 °F/min) ranging from 14°C/min to 2°C/min (90°F/min to 68°F/min). The result is a microstructure with an average hardness of 53 HRC. The other experiment shown in Figure 3 at 1150 °C (2100 °F) has a 118 cm3/s (15 CFH) argon flow rate and an average cooling rate of 2°C/min (36 °F/min) ranging from 7°C/min to 1°C/min (45°F/min to 34°F/min). Comparing these microstructures, there is a secondary austenite phase formation versus the 5 CFH sample microstructure which is entirely pearlite. This secondary phase results in lower average hardness of ~37.5 HRC. We can also conclude that the flow rate of the gas is not dictating the cooling rate for the furnace cool and it may be necessary to investigate further the cause for the differences in cooling rates. The other samples show reasonable correlation between increasing grain size and decreasing hardnesses as well.

Figure 4 Figure 5 Figure 4 and Figure 5 – Microstructures of 4340 wrought steel heat treated and quenched under various conditions Figure 4 shows the micrographs of water quenched wrought 4340 steel. Note the much larger grain sizes found in the samples heat treated in argon versus nitrogen. The nitrogen cover gas may also be introducing nitrides into the steel that promote nucleation and new grain growth resulting in smaller grains. The overall hardness of the samples are very similar due the dominance of the martensite phase in the microstructure so that grain size does not strongly influence hardness. Figure 5 is a set of optical micrographs of 4340 wrought steel sample that have been heated in nitrogen and quenched in oil or water. All samples show crack formation even at the lower 810 °C (1490 °F) for 60 minutes with water quench heat treatment. Martensite is predominant in the microstructures shown in Figure 5 so that the grain size has minimal effect on the sample hardness. The higher temperature heat treatments do result in lower hardness which attributed to carbide formation and reduced martensite transformation during quenching.

The as-quenched AISI 4340 sample indicated martensitic microstructure as indicated by the high hardness and needle-like sub-grain structure. By increasing the tempering temperature to 600°C (1110°F) the microstructure sub-grain structure coarsens and the materials hardness is reduced. AISI 9310-Microstructures

Two sources of 9310 bar stock were provided for testing, Figure 6, with grain sizes in the 50-100 µm range and average hardness between 10 and 25 HRC indicating a low-strength tempered condition [1,2-9]. Coarse lamellae are the most prominent features of the microstructure and support the lower strengths measured than those found in the literature due to tempering at 150 °C - 230 oC (300 °F - 450 °F) . In Figure 7, microstructures of several heat treated samples of 9310 steel are compared based on the time and temperature of the heat treatment in an argon cover gas. Looking at only the large bar material the grain size was observed to increase with longer heating time and higher temperatures. However the increased grain size did not translate to change in the material hardness suggesting the subgrain microstructure dominates the material strength but is not strong affected by this heat treatment indicating a more thermally stable microstructure than observed for 4340 steel shown in Figure 3.

Figure 6 – Optical images of the microstructure Figure 7– Optical images of the of the As-Received 9310 wrought bar stock microstructure of tempered 9310 Steel

under various conditions Figure 8 is a set of micrographs of wrought 9310 steel samples that were heated and water quenched. The micrographs in the upper left and lower right depict clearly the subgrain structures that are observed in all of these samples. These lamellae seem to prefer to nucleate from grain boundaries and protrude into the center of the grain. There is also a fine martensite structure throughout the grain.

Figure. 8 Microstructures of Wrought Figure 9 - Microstructures of Wrough 9310 9310 Steel After Select Heat Treat Cycles Steel After Various Heat Treat Conditions

The hardness of these quenched samples is not strongly dependent on the time and temperature prior to quenching which suggests, as shown in Figure 6, that this material is more thermally stable but with a lower hardness than wrought 4340 steel under the same conditions. There is no indication that there is a strong difference in the effect of argon versus nitrogen on the microstructure of the 9310 wrought steel. The lower micrographs in Figure 9 have slightly larger grain sizes than the initial as-received wrought 9310 steel however the resultant increase in hardness is thought to be due to further formation of the lamellae structures seen in Figure 8. These structures begin to criss-cross as more are formed which is clearly shown in the upper micrograph in Figure 8. This more complicated subgrain structure translates into a harder material. However at higher temperatures this process does not occur probably due to an increase in the grain size which supports the idea that these lamellae tend to nucleate and grain boundaries.

Figure 10 shows the microstructures of the wrought 9310 microstructure after quenching and temper heat treat cycle. Contrary to the results shown for wrought 4340 steel, the lower tempering temperature increases the strength of the steel with a finer and more uniform distribution of lamellae as shown in the bottom two micrographs. The upper two micrographs have a much coarser lamellae and a corresponding lower hardness. Figure 11 is a pair of micrographs comparing similar heat treatments on 9310 wrought steel but with different cover gasses, argon and nitrogen. Both grain size and hardness are similar for both samples confirming that the cover gas is not affecting differently the microstructure. Note also the lack of carbide formation which resulted in an altering effect of the cover gas on the 4340 steel microstructure. Figure 12 shows the density changes of wrought AISI 9310 steel after various process conditions. Figure 13 shows the scaling formation (some scaling thickness reduction due to use of retort) under various heat treat cycle conditions.

Figure 10 Microstructures of wrought AISI 9310 steel Figure 11 Microstructures of After select quench and temper wrought AISI 9310 steel for 3 hrs

Figure 12 Density Changes after Various Heat Treat Cycles of Wrought AISI 9310 Material

Figure 13 Scale Thickness Effects Under Various Heat Treat Cycles

CDC Compacted Materials Behavior and Properties

The springback properties of CDC compaction depend on the % of 9310 added with 4600 series base alloy composition as shown in Figure 14. It was found out that the springback was higher at higher % of gas atomized powder 9310 steel powder addition.

Spring Back from Die; CDC green Tensile Samples

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0 25 50 75 100

Percent 9310 (%)

Chan

ge fr

om D

ie (%

)

W (-635 mesh)L (-635 mesh)W (-325 mesh)L (-325 mesh)W (base as rec.)L (base as rec.)

Spring back from Die; CDC green Tensile Samples (sieved to -325 mesh with fine Ni)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

40 50 60 70 80 90 100

Percent 9310 (%)

Cha

nge from

Die (%

)

WidthLength

Spring back from Die; CDC green Ring Samples

0.000.100.200.300.400.500.600.70

0 25 50 75 100

Percent 9310 (%)

Cha

nge from

Die (%

)

ID (as received)

OD (as received)

ID (sieved to -325mesh with fine Ni)OD (sieved to -325mesh with fine Ni)

Spring back from Die: Green CDC 1" Cylinder Samples (with 4600V seived to -325 mesh with fine Ni)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

50 60 70 80 90 100

Percent 9310 (%)

Cha

nge fro

m d

ie (%

)

150 tsi

105 tsi

85 tsi

Pressed at

Figure 14 Spring Back Characteristics of CDC Compacted Parts as function of Compositions of Alloy Mixes of 4600 series /9310 steels

Also the microstructures/microchemistry (Figure 15 through Figure 17) are reported for vacuum sintered parts (1 inch disks) and microstructures of the optimal tensile sample (#1968) is shown in Figure 18 after vacuum sintering at 1300 °C (2370 °F) and multiple heat treat cycles of HT#29 through #31 (Table 8).

Sample # 1991 Sample # 1993

Figure 15 CDC Compacted and Vacuum Sintered at 1300 °C (2370 °F)-Microstructures °

Figure 16 SEM X-Ray EDS Results of Sample 1989-CDC Compacted Disk @ 150 tsi and Vacuum

Sintered at 1300 °C (2370 °F)

Figure 17 X-Ray EDS Dot Map Results of CDC Compacted Disk Sample # 1989 and Vacuum Sintered at 1300 °C (2370 °F)

The base alloy of 4600 series alone will provide relatively lower strengths after pressing and sintering steps and when mixed with 9310, we have showed improvement for further mechanical strengthening to obtain strengths exceeding 100, 000 psi and above and ductility, under optimal conditions. Table 7 provides various thermal processing parmeters used for sintering and heat treat cycles for CDC compacted samples. The mechanical testing result indicated that we can obtain much higher strengthening and ductility after vacuum sintering at 1300°C (2370°F) followed by multiple heat treatment steps. CDC compacted 50-50 Mix of -325 mesh 4600 series and -325 mesh 9310 after vacuum sintering followed by multiple heat treatments of #29, 30 and 31 is shown in Figure 18. Sample 1968 which is an example of optimally sintered/heat treated materials was also evaluated for improved mechanical properties (Fig. 18, Table 7) such as higher strength and improved ductility, for example, at room temperature as compared to less optimum samples (Tables 6 and 8). Similar encouraging experiments and hot CDC high pressure compaction of mixed and 9310 ferrous alloy powders of various sizes and morphologies are in progress with preliminary HIP equivalent higher part densities (97-99% +)using specialized tooling and additional results will be reported in future publication.

Figure 18 Microstructures of Optimally Processed CDC Sample # 1968 (at Low and High Magnifications -50X and 100X)

Table 6. Mechanical Properties of Sinter-HIPed and Heat Treated Material

Machined Final

Density

Specimen Condition Material UTS

MPa (103 psi)

Strain at Max

Stress (%) g/cm3

Tn-DB21 S&H Cycle 1 4600 series Mix-Base 572 ( 83)

10.20 7.79

Tn-DB22 S&H Cycle 1 4600 series Mix-Base 558 (81)

10.27 7.80

Tn-DB23 S&H Cycle 1 50/50 (-325

mesh) 607 (88)

5.27 7.76

Tn-DB24 S&H Cycle 2 50/50 (-325

mesh) 621 (90)

7.68 7.79

Tn-DB25 S&H Cycle 2 50/50 (-635

mesh) 696 (101)

7.83 7.79

Tn-DB26 S&H Cycle 2 50/50 (-635

mesh) 731 (106)

7.58 7.79

Tn-61 S&HT 1 4600 series Mix-Base 423 (61)

9.23 7.69

Tn-63 S&HT 1 50-/50 (-

325 mesh) 696 (101)

6.98 7.58

Tn-65 S&HT 1 50/50 (-635

mesh) 752 (109)

4.77 7.62

1988 Press and

Sinter 4600 series

Base 450 (65)

11.95 7.70

1989 Press and

Sinter 4600 series

Base 462 (67)

13.44 7.71

Table 7. Experimental Matrix of Heat Treat Cycle Heat Treat Cycle#

Process Conditions Sample ID

25 #25: 1175 oC (2150 F) for 195 min., N2, 25 psi, 30CFH, steel retort 1984, 1986, 1995, 1996, 1997, 1998

26 #26: 810 °C (1490 °F) for 60 min., N2 cover gas, 25 psi, 30CFH, steel retort, Oil Quench

1000C, 9310s-8, , 4340-22, 1984, 1986, 1995, 1996, 1997, 1998

27 #27: 540 °C (1000 °F) for 240 min, slow cool in furnace, N2 cover gas, 25 psi, 30 CFH, steel retort

1000C, 9310s-8, , 4340-22, 1984, 1986, 1995, 1996, 1997, 1998

29 #29: 915 °C (1680 °F) for 60 min., N2 cover gas, 50 psi, 40 CFH, samples removed from furnace & air cooled

1970, 1990, 1992, 1994, R03, R04, R05, 1968, 2009

30 #30: 10oC/min to 840 °C (1540 °F) for 60 min., N2 cover gas, 50 psi, 40 CFH, Oil Quench

1970, 1990, 1992, 1994, R03, R04, R05, 1968, 2009

31 #31: 10 °C/min to 557 °C (1040 °F) for 240 min, N2 cover gas, 40 psi, 40 CFH, slow cool in furnace

1970, 1990, 1992, 1994, R03, R04, R05, 1968, 2009

32 #32: 10 °C/min to 1380 °C (2516 °F) for 240 min, Argon cover gas, 50 psi, 35 CFH, slow cool in furnace

2052, 2053, 2058, 2060, 2061, 2062

33 #33: 10oC/min to 1300 °C (2370 °F) for 240 min, N2 cover gas, 50 psi, 50 CFH, slow cool in furnace

2013, 2015, 2016, 2017, 2063, 2064, 2066

34 #34: 10oC/min to 1300 °C (2370 °F) for 240 min., N2 cover gas, 50 psi, 50 CFH, slow cool in furnace

2075, 2076, 2077, 2078, 2079, 2080

35 #35: 10oC/min to 1300 °C (2370 °F) for 240 min., N2 cover gas, 50 psi, 50 CFH, slow cool in furnace

2067, 2068, 2069, 2070, 2071, 2072, 2073

36 #36: 10oC/min to 1300 °C (2370 °F) for 240 min., N2 cover gas, 50 psi, 50 CFH, slow cool in furnace

2082, 2083, 2085, 2086, 2089, 2090, 2091, 2092

37 #37: 10oC/min to 900 °C (1650 °F) for 60 min., N2 cover gas, 50 psi, 50 CFH, smps removed from furnace while at 900oC & air cooled

2082, 2083, 2085, 2086, 2089, 2090, 2091

38 #38: 10oC/min to 840 °C (1540 °F) for 60 min, N2 cover gas, 50 psi, 50 CFH, Liquid Nitrogen quench for 15 min.

2082, 2083, 2085, 2086, 2089, 2090, 2091

39 #39: 10oC/min to 557 °C (1040 °F) for 240 min., N2 cover gas, 50 psi, 50 CFH, slow cool in furnace

2082, 2083, 2085, 2086, 2089, 2090

Table 8. Mechanical Property Results (Using Instron 5500 Series Electro-Mechanical Tester)

Sample ID

Green Density (g/cm3)

Final Part

Density (g/cm3)

Max Load kg (lbm)

Load at Break

kg (lbm)

Ultimate Tensile Strength

MPa (103 psi)

Extension at

Breakage (%)

Comments

1968* 7.53 7.64 2612 (5753) 2027 (4465) 734 (106) 23.28 HT#29, #30, #31; 50-50 Mix (-325 mesh 4600 series and -325 mesh

9310); 15-16 HRC 2009 7.45 7.14 2027(4465) 1153 (2540) 535 (77) 12.85 HT#29, #30, #31 1970 7.37 7.13 1643 (3620) 930 (2048) 434 (63) 11.74 HT #29, #30, #31 2058 7.19 7.46 609 (1341) 248( 548) 262 (38) 16.6 HT # 32 2053 7.25 7.62 659 (1451) 546 (1202) 257 (37) 18.02 HT # 32 2015 7.51 1878 (4138) 1041 (2293) 558 (81) 5.00 HT # 33 2016 7.51 1299 (2861) 472 (1039) 359 (52) 14.20 HT # 33 2063 6.99 7.09 1068 (2353) 582 (1282) 297 (43) 9.67 HT # 33 2075 7.40 7.59 1178 (2595) 987 (2175) 427 (62) 37.80 HT # 34; 50-50 Mix (+325 mesh 4600

series and -325 9310) 2092 7.38 7.66 1269 (2795) 828 (1825) 304 (62) 36.00 HT#36; 50-50 Mix (-325 mesh 4600

series and -635 mesh 9310) 2091 7.24 7.57 1488 (3278) 1315 (2897) 517 (75) 34.90 HT#36, #37, #38; 50-50 Mix (-325

mesh 4600 series & -635 mesh 9310 ) 2083 7.17 7.49 1428 (3147) 1380 (3040) 533 (77) 24.4 HT#36-39; 50-50 Mix(-325 4600

series & -635 mesh 9310); 85 HRB 2090 7.20 7.59 1227 (2704) 1066 (2349) 509 (74) 33.7 HT#36-39; 50-50 Mix (-325 mesh

4600 series and -635 mesh 9310); ~76-88 HRB

2082 7.40 7.69 1576 (3471) 1364 (3004) 572 (83) 30.20 HT#36-39; 91 HRB 2085 7.40 7.64 1325 (2919) 1149 (2531) 558 (81) 29.5 HT#36-39; 86-88 HRB 2086 7.14 7.47 892 (1965) 832 (1834) 353 (51) 40.30 HT#36-39; -325 mesh 4600 series ; 52

HRB 2089 7.54 7.82 909 (2003) 718 (1582) 379 (55) 59.8 HT#36-39; +325 mesh 4600 series;

57-62 HRB

Summary of Conclusions:

• CDC high pressure ~2100 MPa (150 tsi) compaction has been used to compact a variety of low-alloy steel mixes using water-atomized 4600 series base powder and gas-atomized 9310 steel powder for improved densification and properties than possible by conventional low pressure PM methods.

• Select thermal processing of CDC compacted samples such as vacuum sintering and multiple heat treat cycles (Table 7) has been optimized to obtain better densification together with improved fine microstructures and mechanical properties such as strength and ductility.

• SEM and X-ray EDS results of vacuum sintered microstructures revealed fine grained morphologies (Figure 16 through Figure 18) and suitable chemistry.

• Vacuum sintering followed by multiple heat treatment steps were found to be beneficial for improving the mechanical properties of higher strength together with especially much better ductilities (Table 8) of CDC high pressure compacted PM parts.

• The springback properties (Figure 14) were found to changes as a function of alloy composition. • The heat treat cycles and materials behavior including microstructures, density changes,

macrohardness and scaling behavior of wrought steels such as AISI 9310 and AISI 4340 steels provided some baseline guidelines and establishment of suitable heat treat conditions of temperatures, environment, quenching media, and process times, as needed for heat treating the high pressure compacted CDC low-alloy steel materials for optimizing the properties.

Acknowledgements

We like to thank our project sponsors from DOD SBIR/STTR programs such as NAVY STTR, MDA, DOE and NASA for the Materials and Manufacturing Aspects of Combustion Driven Compaction Technology. For the CDC ferrous alloys program, We like to thank Dr. Charles Lei of NAVAIR (under NAVY STTR Phase II Contract # N68335-07-C-0144) for his technical monitorship and support. We also appreciate the technical assistance of Matt Bednar, Donald Trostle, Valeri Kuzin and Kevin Mcmohan in this project. References

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3. Townsend, D.P., Turza, A., Chaplin, M., The Surface Fatigue Life of Contour Induction Hardened AISI 1552 Gears, NASA Technical Memorandum 107017, Army Research Laboratory Technical Report ARL-TR-808,(1995)

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