NITROGEN ALLOYING OF PM STEELS: PROCESSING
AND PROPERTIES
Chris Schade & Tom Murphy
Hoeganaes Corporation
Cinnaminson, NJ 08077
Alan Lawley & Roger Doherty
Drexel University
Philadelphia, PA 19104
ABSTRACT
A process has been developed to introduce nitrogen as a bulk alloying
element in low alloy PM steels. The use of nitrogen to replace carbon as an
alloying element to develop Fe-N martensite instead of Fe-C martensite is of
interest since it has been suggested that, in ingot steels, at a given strength
level, Fe-N martensite exhibits enhanced ductility, compared with Fe-C
martensite; nitrogen promotes the formation of tough lath at the expense of
brittle plate martensite. Sintering and post sinter processing are detailed in
relation to attendant microstructures and mechanical properties of PM Fe-N
and equivalent Fe-C alloys, in particular strength and ductility. The
hardenability of Fe-N, Fe-C and Fe-N-C alloys was also determined utilizing
the jominy end quench test. In 4100 alloys the hardenability of Fe-C was
superior to that of Fe-N and Fe-C-N.
INTRODUCTION
There is evidence that iron-nitrogen alloys can exhibit superior combinations
of strength and toughness, when compared with the equivalent iron-carbon
system. This has been attributed to the transition from tough lath martensite
to brittle plate martensite at higher atomic interstitial content in iron-nitrogen
compared with iron-carbon alloys. The higher nitrogen content leads to
improved interstitial solid solution strengthening, and since the transition to
plate martensite occurs at higher nitrogen contents, the strength and ductility
combination may be increased.
Melting and atomization cannot be used to make iron-nitrogen alloys, since
nitrogen can only be kept in solution at low temperatures. At high
temperatures, particularly above the melting point, excessively high pressures
are required for even a small concentration of nitrogen in solution [1]. The bulk
nitrogen concentration in iron at 0.1 MPa (14.7 psi) is < 0.05 w/o. Nitrogen
solubility in both liquid and solid iron can be increased by melting under
nitrogen pressure, for example, 1.5 w/o nitrogen at 200 MPa nitrogen
pressure [2-3]. Since the solubility of nitrogen gas also increases with
temperature, it is difficult to produce material without excessive porosity and
the cost of production precludes using this method. It is this limitation that has
prevented the production of bulk iron-nitrogen alloys.
Currently iron-nitrogen alloys are limited to surface-nitrided layers by
interaction with gases at various nitrogen equivalent pressures- usually
ammonia/hydrogen mixtures (NH3/H2). Mittemeijer et al. reported nitriding
iron to a depth of 400 µm in 12 h at 810 oC, with longer times at that
temperature leading to the precipitation of free nitrogen as voids [4].
Commercial methods commonly used are gas-phase-nitriding and plasmaion-nitriding [5]
While the introduction of nitrogen at the surface of iron-alloys has benefits
such as increased hardness and wear resistance the kinetics of nitrogen
diffusion and the cost of these process make them unsuitable in the bulk
alloying of nitrogen. The purpose of the present work was to develop a PM
process for bulk nitrogen alloying. The mechanical properties of Fe-C PM
alloys are compared with those of equivalent Fe-N alloys and Fe-C-N alloys in
relation to microstructures, mechanical properties and hardenability.
ALLOY PROCESSING AND TESTING
To produce bulk samples with nitrogen alloyed throughout the entire thickness
of the specimen, a new approach using conventional PM processing was
developed. One of the benefits of PM is that various additives can be mixed
with the base powder and during sintering these elements diffuse into the
matrix alloying the bulk of the specimen (or part). Given sufficient time at
temperature this approach can produce a homogenous alloy. This
“Inside/Out” approach was used in the present study to produce homogenous
nitrogen containing alloys.
It is common in PM applications to use ground ferroalloys mixed with base
iron powders to produce alloy powders [6]. Materials include ferrochromium,
ferromanganese and silicon containing ferroalloys. The dissolution of the
particles and diffusion of the alloying elements normally depends on time at
temperature but practical experience has suggested that particles with a
mean particle size (d50) of 8 to 10 µm can be processed at reasonable
sintering temperatures (1120 oC to 1260 oC, 30 min).
This same concept can be used with nitrogen containing ferroalloys. Table I
shows the composition of the three additives used in the present study. All
three are available commercially in the required particle size distribution for
PM applications. In the “Inside/Out” approach, the additive particles are
blended with a base powder, compacted, and during sintering the nitrogen
(and other alloy elements) diffuse into the matrix. Since, the additives are
distributed throughout the specimen, the nitrogen is present homogenously
throughout the thickness of the specimen after sintering.
Table I: Composition of additive particles containing nitrogen. (w/o).
In order to examine the influence of nitrogen on mechanical properties, two
sets of alloys were prepared with compositions close to AISI 4140 (Table II).
The first PM alloy, 4140C, was made by admixing appropriate levels of
ferrosilicon, high carbon ferrochromium and ferromanganese with a
prealloyed powder containing 0.30 w/o Mo (Ancorsteel 30HP). Additional
graphite was added to achieve a sintered carbon content of 0.40 w/o. This
alloy was used as the baseline for the study. The second alloy, 4140N, was
prepared in a similar fashion: however, silicon nitride, chromium nitride and
manganese nitride were added to the 0.30 w/o Mo powder. After sintering at
1120 oC for 30 min, this alloy contained 0.46 w/o nitrogen and only trace
amounts of carbon (< .03 w/o).
Table II: Composition of Experimental Alloys (w/o).
TR specimens were compacted at 690 MPa from the two compositions and
sintered in a continuous belt furnace at 1120 oC for 30 min, in an atmosphere
containing 90 v/o nitrogen and 10 v/o hydrogen. Accelerated cooling was
utilized in order to aid the transformation to martensite.
In order to assess the hardenability of Fe-C versus Fe-N in the 4100 alloy
system, Jominy end quench samples were prepared and hardness profiles
determined on samples austenitized at 900 oC and water quenched using
standard procedures.
Microstructures were characterized by optical microscopy utilizing specimens
prepared by standard metallographic techniques. The microstructures were
examined in the unetched and etched (2 v/o Nital - 4 w/o Picral) conditions.
RESULTS AND DISCUSSION
Mechanical Properties
TR test results are shown in Table III. The 4140 alloys exhibited the expected
apparent hardness and TR strength, based on the alloy content utilized. In
contrast, 4140N, which contained an equivalent amount of alloy content, did
not exhibit the strength or apparent hardness expected.
Table III: Transverse Rupture Properties of Experimental 4140 Alloys.
In order to determine the reason for the low mechanical properties, the
microstructures of the two alloys were compared (Figure 1). The
microstructure of the 4140C consisted of lath martensite, consistent with its
mechanical properties. The microstructure of the 4140N exhibited a mixture
of pearlite and ferrite, notwithstanding the fact that the ferroalloy particles
appeared to have diffused into the matrix. The nitrogen level of the alloy was
confirmed by Leco combustion analysis (0.46 w/o). It would appear that the
cooling rate resulting from the accelerated cooling in the sintering furnace was
sufficient to transform the carbon-bearing 4140 to martensite, but not the
nitrogen-bearing alloy.
(a)
(b)
Figure 1. Representative microstructures: (a) 4140C -lath martensite (b) 4140N- pearliteferrite
Tensile properties of the Fe-N and Fe-C alloys are shown in Table IV.
Examination of the microstructure of oil quenched 4140N revealed a
microstructure of lath martensite similar to the 4140C. Despite the similar
microstructures, the 4140C exhibited a higher apparent hardness, indicating
that the hardenability of Fe-C is higher than that of Fe-N. The 4140N exhibits
a surprisingly high level of ductility in relation to yield strength.
Table IV: Tensile Properties of Experimental 4140 Alloys.
Hardenability
Figure 2 shows the hardness profiles from three Jominy end quench
specimens with low levels of interstitials (< 0.45 w/o); (i) carbon only (0.42
w/o), (ii) nitrogen only (0.25 w/o) and (iii) carbon and nitrogen ( C+N = 0.38
w/o). Figure 3 shows similar data fort interstitial contents > 0.60 w/o.
(a)
(b)
Figure 2. Hardenability of 4100 PM Alloys with interstitial elements: (a) < 0.45 w/o and (b) >
0.60 w/o.
The hardness profile of the Fe-C alloys was superior to that of the Fe-N alloys
at low and high carbon levels. However, when carbon and nitrogen are
combined, after an initial drop in hardness the hardness profile levels off to a
stable value relatively quickly. EDX analysis, Figure 3 reveals about the same
level of undissolved alloy particles in the Fe-C and Fe-N alloys. However,
since the nitrogen is contained within the alloy particles, full dissolution of the
particles is necessary to diffuse the nitrogen into iron, whereas in the Fe-C
alloys carbon is added in the form of graphite which goes into solution directly.
This accounts for the Fe-C microstructure being a mixture of martensite and
bainite while the Fe-N is composed of a martensite/ferrite mixture. To get the
full effect of nitrogen, the alloy particles must be fully dissolved during
sintering.
Figure 3. Optical micrograph showing undissolved alloy particles and EDX (SEM) results of
4140C and 4140N for chromium, manganese and silicon.
INTERPRETATION
From the observations and results, it is concluded that the use of nitrogen to
replace all the carbon in an alloy is not justified, due to the extra processing
steps required to achieve hardenability. However the higher ductility at
equivalent strengths does suggest some advantages to the use of nitrogen as
an alloying element. The literature on carbonitriding suggests that nitrogen
facilitates the dissolution of carbon in iron and will contribute to a significant
increase in the kinetics of the diffusion process; thus nitrogen in the diffused
layers will enhance the activity of carbon in austenite [7]. Both these factors
will move the isothermal cooling curve to the right and may results in a
synergistic effect of carbon and nitrogen on the martensite transformation.
(a)
(b)
Figure 4. Comparison of tensile properties: (a) yield strength, and (b) elongation for 4140C
(carbon only) and for 4140C +N (carbon + nitrogen). For carbon + nitrogen alloys - interstitial
content = 0.2 w/o N + X w/o C.
It has also been found that the transition from lath to plate martensite occurs
at higher levels of nitrogen, leading to a superior combination of strength and
toughness. This suggests that a combination of carbon and nitrogen may lead
to superior properties. To examine this possibility, an alloy of 4140C+N was
produced in which the nitrogen level was held constant at 0.20 w/o and
various levels of carbon were added in the form of graphite. The nitrogen was
added by the method outlined previously.
Figure 4 shows the yield strength and elongation of a 4140 alloy with a
combination of carbon plus nitrogen, in excess of the levels in 4140 using only
carbon. Both sets of alloys were pressed at 690 MPa and sintered at 1120 oC
in 90 v/o nitrogen/10 v/o hydrogen. The sintered density of the two alloys was
equivalent. Both groups of specimens were austenitized at 900 oC and oil
quenched. The 4140C yield strength and ductility drop off dramatically at ~0.5
w/o C. In comparison, the 4140C+N has a much more gradual drop off in
properties. The reason for the improved properties is clear from Figure 5
which shows that the transition from lath to plate martensite occurs at higher
levels of interstitial content (carbon and nitrogen) in the 4140C+N alloy. Since
the transition from lath to plate occurs at a higher interstitial content, solid
solution strengthening is increased. Note that this increase in strength does
not result in a decrease in ductility of 4140C+N.
Figure 5. Microstructure of 4140C+N versus 4140C showing the levels of lath and plate
martensite.
CONCLUSIONS
•
Nitrogen can be used as a replacement for carbon in PM alloys but
requires special processing to be effective.
•
In Fe-C the microstructure is a mixture of martensite and bainite,
whereas Fe-N exhibits a mixture of martensite and ferrite.
•
Fe-N exhibits a superior strength to ductility ratio compared with Fe-C.
•
The hardenability of PM Fe-N is lower than that of PM Fe-C.
•
There is a synergistic effect of nitrogen in the presence of carbon in
iron in relation to diffusion and the martensite transformation.
•
The hardenability of Fe-C is superior to that of Fe-C-N and Fe-N.
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