PRE-ALLOYED BORON IN POWDERED METAL (P/M) STAINLESS STEELS
Chris Schade and John Schaberl
Hoeganaes Corporation
Cinnaminson, NJ 08077
Sanjay N. Thakur and Vaidehi C. Dongre
Hazentec, L.L.C.
Hazentec, AR 72064
ABSTRACT
The demand for high-density stainless parts continues to grow as P/M continues to battle
conventional stainless. The compressibility and hence the final sintered density of P/M
stainless steels are limited due to their high alloy content. The addition of boron allows
P/M to produce nearly full density parts that achieve mechanical properties similar to
wrought materials. By adding boron to the melt prior to atomizing, the segregation of
boron is minimized leading to a more uniform microstructure and properties. The present
work will examine the role of pre-alloyed boron in both austenitic and ferritic stainless
steels. Experimental work will involve the measurement of green and sintered properties.
Using the laboratory experiments as a guide, specimens will be sintered in production
furnaces to determine the performance of various grades of boron containing stainless
steels. The specimens will be evaluated for density distribution, dimensional stability and
mechanical properties.
INTRODUCTION
The use of boron in P/M stainless steels has proven to be an excellent method to improve
the sintered density of the final part.1-3 Boron can be added in the form of elemental
powders or pre-alloyed in the steel prior to atomizing. In both cases the boron forms a
low melting eutectic that allows for liquid phase sintering. The pore structure of a
conventional P/M 316L stainless steel is compared to that of a pre-alloyed 316L with
.15% boron in Figure 1. The need to control boron levels accurately is important because
excessive boron leads to precipitates or excessive eutectic phase at the grain boundary,
both of which will embrittle the part and deteriorate performance. Previous work has
suggested that boron levels of 0.15 to 0.25 weight percent can be used favorably in 316L
to enhance mechanical properties. Table I shows that the physical properties of the P/M
316L with boron approaches those of the wrought version.
(a)
(b)
Figure 1. Porosity distribution in (a) 316L with boron and (b) conventional 316L.
Prasan et al4, have shown that high densities achieved by using boron in 316L leads to
low levels of crevice corrosion for the P/M steels. Other studies5 have shown that the salt
spray corrosion resistance of the 316L with boron out-performs P/M stainless steels
designed for improved corrosion resistance (such as 316L with copper and tin additions).
These works have shown that as the interconnected porosity is eliminated in the P/M
steels, most forms of corrosion resistance improve.
Table I. Tensile properties of P/M 316L with boron and wrought 316L.
Pressure
(MPa)
(tsi)
Material
Wrought 316L 1
P/M 316L with .25% Boron
690
1
50
UTS
(MPa) (103 psi)
483
70
490
71
0.20% OFFSET Elongation
(MPa) (103 psi)
(%)
172
220
25
32
30
28
ASTM A479 (Specification for Annealed Bar)
Despite all the benefits shown by the use of boron in P/M 316L there has been little
commercial acceptance of the grade. As shown by previous researchers, the processing
of alloys using liquid phase sintering requires an understanding of the alloy system and
processing parameters such as compaction characteristics, heating rate, sintering
temperature and atmosphere.6-12 Powder characteristics such as particle size and shape
can also play an important role in the final density of the P/M part.
EXPERIMENTAL PROCEDURES
Most of the previous work using boron has focused on the use of elemental boron or
boron-containing compounds (i.e. ferro-boron or nickel boron) admixed to water
atomized stainless steel powders. Most of the studies have focused on using 316L
stainless steel as the base powder. While these methods have proven effective, the
advantages to pre-alloying the boron in the melt prior to atomization include cost savings
and a more uniform distribution of boron in the powder. In order to understand the
manufacturing of stainless steel powders with pre-alloyed boron, four grades of stainless
steel were produced. To determine the amount of boron to put in the pre-alloys a series
of 316L powders with various boron levels were sintered at various temperatures in a
hydrogen atmosphere. 316L was chosen because of the successful history of liquid phase
sintering with the alloy. The results of these tests are shown in Table II. The sintering
temperatures were chosen because they represent a realistic target for production
furnaces. From this work it was determined that the optimum boron content for the prealloys was between 0.15 and 0.25%.
Table II. Optimization of sintered density for 316L pre-alloyed with boron.
Temperature
Sintered Density (g/cm3)
(oC)
1232
1246
1260
1274
(oF)
2250
2275
2300
2325
0% Boron 0.05% Boron 0.15% Boron 0.25% Boron
7.00
6.99
7.26
7.60
7.02
7.00
7.36
7.69
7.04
7.02
7.50
7.72
7.06
7.09
7.77
7.74
0.75% Acrawax C used as lubricant and Compacted at 690 Mpa (50 tsi).
Much of the current knowledge on boron containing stainless steel has been developed
using 316L. In order to determine if boron can be used as a general additive several
grades of stainless steel were produced. These include 409CB, 410L, 316L and a highly
alloyed 316L (Table III). The highly alloyed 316L is a composition studied in an early
patent by Reen.3 The author has suggested that the eutectic that forms the liquid phase
may be influenced by the base chemistry of the powder. The significant levels of
molybdenum and nickel in this alloy may lead to a different composition of eutectic than
the other alloys; therefore it was included in this study. 409Cb and 410L are commonly
used in P/M and should form a simple iron-chromium eutectic.
Table III. Chemical Composition of pre-alloyed boron powders.
Type
409Cb
410L
316L
316L PLUS
C
(%)
0.013
0.021
0.014
0.016
S
O
(%)
(%)
0.007 0.25
0.008 0.14
0.008 0.150
0.006 0.46
N
(%)
0.013
0.003
0.019
0.022
P
(%)
0.013
0.008
0.015
0.015
Si
(%)
0.80
0.80
0.83
0.80
Cr
(%)
12.00
11.91
17.30
18.60
Ni
(%)
0.04
0.06
13.10
18.40
Cu
(%)
0.03
0.03
0.03
0.03
Mn
(%)
0.11
0.20
0.12
0.14
Mo
(%)
0.07
0.02
2.31
3.70
Cb
(%)
0.40
-------
B
(%)
0.15
0.17
0.24
0.17
All powders were water atomized and screened to a nominal –100 mesh. The powders
had a minus 325-mesh content of 20 to 35% with apparent densities between 2.85 and 3.0
g/cc. In future discussion the boron containing grades shown in Table III will be
designated as 316LB, 410LB, 409LB and 316LB Plus.
RESULTS AND DISCUSSION
The pre-alloyed boron powders processed very similarly to normal water atomized
stainless steel powders. The only major differences were in the materials green density
and green strength. Figure 2 shows the green density of 409LB and 316LB over a range
of compaction pressures. The 409LB is compared to the conventional 409Cb and it can
be seen that the green density decreases significantly (0.15 g/cm3) when boron is added.
The 316LB was compared to a conventional 316L with 0.20 weight percent Ferro-boron
admixed. The addition of Ferro-boron to conventional 316L has little or no effect on
compressibility, but the addition of boron as a pre-alloy drops the green density by
approximately .30 g/cm3 at a given compaction pressure. It is evident that the boron in
the pre-alloys actually acts as a solid solution strengthener and the powder is not as
compressible as when the boron is added as a pre-mixed additive. The lower green
strength of the pre-alloyed boron materials appears to be related to the lower green
density since the powder shape and apparent density were very similar to the same grades
without the pre-alloyed boron.
Compaction Pressure (MPa)
345
395
445
495
545
595
645
695
745
7
Green Density (g/cm3)
6.8
6.6
6.4
6.2
6
5.8
5.6
25
30
35
40
45
50
55
Compaction Pressure (tsi)
409LB
409CB
316L
316L with FB
Figure 2. Green density of powders with and without boron additions.
The role of boron in the densification of the four alloys is evident by their high sintered
density as shown in Table IV. For grades without boron, under normal processing
conditions, ferritic grades achieve around 94% of theoretical density while the austenitic
316L achieve around 90% of theoretical density. It appears from this work that a wide
range of stainless PM alloys can take advantage of liquid phase sintering with prealloyed
boron.
Table IV. Sintered density of boron prealloys.
Pressure
Material
409LB
410LB
316LB
316LB Plus
(MPa)
(tsi)
690
690
690
690
50
50
50
50
Sintered
Density
Theoretical
Density
(g/cm3)
7.55
7.66
7.68
7.73
(g/cm3)
7.7
7.7
8.0
8.0
Percent of
Theoretical
%
98
99
96
97
0.75% Acrawax C used as lubricant and sintered at 1260 oC (2300 oF) in 100% Hydrogen.
Sintering Temperature
It is well known in the P/M industry, that as the density of the part increases so do all of
its mechanical and corrosion properties. In general, higher sintering temperature leads to
higher sintered densities and therefore better mechanical properties. In order to form the
liquid phase the temperature of the part must exceed the solidus temperature. However,
above the solidus temperature there is a temperature at which the optimum amount of
liquid is present. Below this temperature there is not enough liquid to aid in densification
and above this temperature the excess liquid swells or distorts the part (commonly called
slumping). Higher temperatures also lead to grain growth and micro-structural changes
that can lead to poor mechanical properties. Unlike conventional sintering, with the use
of boron there is an optimum sintering window that must be targeted. Figure 3 shows the
sintered density of the various pre-alloyed boron grades versus sintering temperature.
Above 1274 oC (2325 oF) the hardness of the materials started to decline indicating grain
coarsening. Also, as will be discussed later, the dimensional stability of the alloys started
to degrade as measured by rectangularity of the TRS specimens. Above a sintering
temperature of 1232 oC (2250 oF) the transverse rupture bars were too dense and ductile
to break and simply bent. Therefore, final density was used as the measurement for
degree of sinter.
Temperature (C)
1225
1230
1235
1240
1245
1250
1255
1260
1265
1270
1275
1280
7.85
Sintered Density (g/cc)
7.8
7.75
7.7
7.65
7.6
7.55
7.5
7.45
7.4
2240
2250
2260
2270
2280
2290
2300
2310
2320
2330
Temperature (F)
409LB
316LB
316LB Plus
410LB
Figure 3. Sintered density of boron containing grades versus sintering temperature.
Particle Size Distribution
Table II shows the temperature window to achieve high densities can be lowered and
widened with increasing boron levels. Several investigators have suggested that finer
particle sizes would lower the temperature window for densification.6, 13 In fact, most tool
steels that are liquid phase sintered use powder which is finer then the normal –100 mesh
powder used in stainless steels. Technology is being developed that allows for the
agglomeration of fine powders that then can be used for press and sinter applications. In
order to determine the effects of particle size on the sintering window, a batch of powder
with the same chemistry as the 316L with boron shown in Table III but with a finer
particle size was produced. This material was then mixed with the coarser –100 mesh
powder to form the particle size distributions shown in Table V. In addition, the fine
316L with boron was agglomerated with polyvinyl-alcohol at a concentration of 0.15%.
This agglomerated powder was then mixed with 0.75% Acrawax C and pressed and
sintered in the same fashion as normal molding grade powders. Figure 4 shows the
sintering response for these materials as a function of sintering temperature.
Table V. Particle size distributions of 316L with boron (in micrometers).
Material
Fines
PSD1
PSD2
-100
Agglomerated
Particle Size Distibution of 316LB
D50
D90
D10
14.60
34.40
68.60
21.60
50.60
118.10
23.10
52.20
120.50
29.00
60.90
119.10
16.20
37.60
78.80
Temperature (C)
1140
1160
1180
1200
1220
1240
1260
1280
8
Sintered Density (g/cm3)
7.8
7.6
7.4
7.2
7
6.8
6.6
2050
2100
2150
2200
2250
2300
2350
Temperature (F)
316L 25/75
316L 50/50
Agg. 316L
316L
Figure 4. Effect of particle size on sintered density of boron containing 316L.
From the results in Figure 4 it can be seen that the particle size distribution can lower the
sintering temperature by 25 to 50 degrees. However consideration should be given to the
potential complications of compacting finer powders, such as tool wear and ejection
forces.
Compaction and Distortion
In order to achieve high sintered densities the green density of the compact must be high
enough to prevent excessive shrinkage. The green density must also be uniform so that
part distortion is kept to a minimum. Table VI shows the green density, sintered density,
shrinkage and distortion measured as a function of compaction pressure. The distortion
was measured as the difference in length of the opposing sides of a 3.18 cm (1.25-inch)
long transverse rupture specimen. In all cases a high compaction pressure was necessary
to achieve close to full density. The boron containing grades are lower in compressibility
than the standard grades due to the solid solution strengthening of the boron. However,
despite this fact, high sintered densities and reasonable dimensional change can be
achieved at a reasonable compaction tonnage (690 MPa or 50 tsi). Even at the lower
compaction tonnages the sintered density of the boron containing grades is significantly
higher than the same grades without boron. However, the large shrinkages may be
difficult to deal with in a production setting.
The distortion of the transverse rupture specimens seemed to be insensitive to compaction
tonnage. The austenitic grade of 316LB had the least distortion while the ferritic grades
had the most distortion. Previous authors have noted that carbon levels in boron
containing stainless are generally higher due to the reduced level of oxygen (which reacts
with the boron). 4 In austenitic stainless steels, this level of carbon will not impact the
microstructure. However, there may be enough carbon in the ferritic grades to form
martensite. This could possibly explain the higher distortion values for the ferritic
grades. This could be further supported by the fact that the distortion does not change
with compaction pressure.
Table VI. Sintered density and distortion versus compaction tonnage.
Pressure
Distortion
(HRB)
(mm)
(in)
30
-6.79
52
0.1270
0.005
550
40
7.39
-6.24
79
0.1016
0.004
690
50
7.55
-5.66
80
0.1270
0.005
415
30
7.17
-7.42
78
0.3048
0.012
550
40
7.40
-6.42
92
0.3556
0.014
690
50
7.66
-6.19
98
0.3556
0.014
415
30
7.26
-6.27
54
0.0254
0.001
550
40
7.51
-5.55
64
0.0254
0.001
50
7.68
-4.99
67
0.0508
0.002
(tsi)
409LB
415
690
o
Hardness
(%)
(MPa)
316LB
D.C.
(g/cm3)
7.09
Material
410LB
Sintered
Density
o
Sintered at 1260 C ( 2300 F), in 100% hydrogen.
Furnace Atmosphere and Heating Rate
It has already been shown that boron containing stainless steels must be sintered in pure
hydrogen in order to achieve high densities.12 Even atmospheres containing as little as
25% nitrogen leads to the formation of boron nitride precipitates which impede sintering.
As has been previously noted, boron grades of stainless steel generally have higher
carbon levels and therefore the carbon potential of the furnace should also be kept low.
Three types of furnaces were used in this study, mainly to determine the effect of heating
rate (discussed below). The first furnace was a continuous belt furnace, which had a dew
point of –40 oC and was run with a pure hydrogen atmosphere. The only nitrogen in the
system was a nitrogen curtain at the end of the furnace used to prevent air ingress. The
nitrogen flow rate was such that this accounted for only 2% of the furnace atmosphere
(the balance being hydrogen). Despite this low flow rate, the test specimens finished
with nitrogen contents above 0.1%. This led to poor sintered density and poor
mechanical properties. The second furnace was a pusher-type furnace with a 100%
hydrogen atmosphere, but a slightly higher carbon potential. No nitrogen was required to
purge the furnace exit due to mechanical seals. The nitrogen contents of the parts
sintered in this furnace were around 0.04%. Due to the higher carbon potential in this
furnace the ferritic grades had a final sintered carbon of around 0.05%. Finally, the parts
were sintered in continuous furnace that had a low dew point, low carbon potential and
100% hydrogen atmosphere. These parts exhibited low carbon and low nitrogen
contents, however the final density of the parts was effected by the heating-rate
(discussed below). Table VII summarizes the results of the furnace atmosphere on the
carbon, oxygen and nitrogen of the parts.
Table VII. Typical carbon, nitrogen and oxygen levels for various test furnaces.
Type
Carbon
(%)
Nitrogen Oxygen
(%)
(%)
Furnace 1
0.020
0.100
0.25
Furnace 2
0.060
0.050
0.30
Furnace 3
0.014
0.008
0.33
Description
Continuous, Nitrogen Safety Curtain, Low
Dew-Point-Low Carbon Potential
Pusher, 100% Hydrogen, High Carbon
Potential
Continuous, No Nitrogen, Low Dew-Point
Low Carbon Potential
It has been suggested that the heating rate can impact the liquid phase sintering response
of a material.6 In the case of boron, which tends to be a surface-active element, slower
heating rates allow for homogenization of the alloy. This leads to fewer compositional
gradients and can lead to slower diffusion rates. This results in less densification. The
furnaces previously described were used to determine the effects of heating rate on final
density. Furnace 2 was used to create two different heating rates as shown in Table VIII.
Both materials were heated to 1260 oC (2300 oF) and held at temperature for 45 minutes.
It can be seen that the slower heating rate led to a lower final sintered density,
presumably from the homogenization of the boron. This was also supported by a
sintering trial performed in the production furnace (Furnace 3). The material was heated
to a temperature of 1288 oC (2350 oF) and held for 45 minutes. According to Figure 4,
this should have led to a higher sintered density for all the various grades, but Table VIII
shows that the densities of the materials in the production furnace are far less then
expected despite low levels of carbon and nitrogen. Therefore it appears that heating rate
plays an important role in the final densification of the materials.
Table VIII. The effect of heating rate on sintered density.
Heating Rate
Sintered Density
(oC/sec)
1.4
(oF/sec)
2.5
(g/cm3)
7.68
0.28
0.5
7.43
PROD.
PROD.
7.38
410LB
PROD.
PROD.
7.32
409LB
PROD.
PROD.
7.24
316LB Plus
PROD.
PROD.
7.63
Material
316LB
CONCLUSIONS
•
Both austenitic and ferritic stainless steels can be pre-alloyed with boron to
achieve high sintered densities (typically 98% of theoretical).
•
In order to maximize green density, sintered density and green strength the boron
level should be kept between 0.15 and 0.25% percent.
•
High sintered densities of boron containing stainless steels can be achieved at
temperatures typically used to process conventional stainless steels.
•
A finer particle size distribution can lower the sintering temperature necessary for
densification.
•
A finer particle size distribution can also increase the sintered density at a given
sintering temperature.
•
Compaction tonnages of 50 tsi are required to maximize density of boron
containing stainless steels.
•
Distortion of test specimens was dependent on the grade of stainless steel.
•
To maximize sintered density a furnace atmosphere that is free of nitrogen, has a
low carbon potential and a low dew point is needed.
•
Heating rate of the sintering furnace impacts the final sintered density in boron
containing alloys. Faster heating rates are necessary to achieve high sintered
densities.
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