Mechanical Properties of High Density Low Alloyed PM Steels: Effect of Sintering and Secondary Heat Treatments

Theoretical Remarks 
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Density, Porosity and Load-bearing section 
The properties of sintered steels can approach those observed via other 
metalworking techniques [2]. One difference is with the property decrements due 
to porosity. 
Thus, as a first point in understanding properties of PM materials there is a 
requirement that porosity (density) and pore structure differences are 
measurable. Creating a favourable pore shape depends on processing conditions. 
Generally, those actions that improve green density and sintering are favourable 
to the mechanical properties. At low densities, the strength is limited by the 
interparticle bond size. For the higher densities, the strength has a complex 
dependence on the pore structure and particle boundaries. 
Pores degrade the mechanical properties, partly because of the reduced 
area supporting the load; but main factor is the stress concentration at each 
pore. 
The porosity, or the fraction of empty space in the sample, is the parameter 
which is most commonly used in order to characterize sintered materials. With 
conventional PM parts which do not exhibit pronounced shrinkage during 
sintering, total porosity is primarily a function of compacting pressure. The value 
of total porosity is obtained by means of density measurements, often carried 
out using the water displacement method [3]. 
It is known that the porosity is not uniformly distributed in a pressed and 
sintered samples. The distribution of porosity within the specimen volume may 
be determined from metallographic sections by using stereological techniques. 
The distance between individual pores is mainly a function of the particle 
size of the starting powder. Not only the porosity but also the pore spacing may 
vary within the sample volume. As a result of the particle size distribution in the 
starting powder, areas with increased content of finer particles will result in more 
numerous – tough finer – pores in compacted specimen. 
Pores may be interconnected or isolated. In the case of fully interconnected 
porosity , one single “pore”, in the form of the pore network, should exist in the 
sample. Here the material consist of two intertwined continuous phases – the 
metallic and the pore phase. In the case of isolated porosity, the pores are 
presented as a dispersed phase in a solid matrix. In practice, the interconnected 
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or isolated pores are often regarded as being equivalent to open or closed 
porosity, respectively. The open pores intersect the free surface of the specimen 
and can easily be determinate by infiltration methods. It is frequently assumed 
that interconnected porosity or pore channels are always open to the infiltrating 
liquid. This is not necessarily true, however, since there might be islands of 
interconnected pores in the interior of a specimen which is inaccessible from the 
outside (and should therefore be termed “closed” pores). The ratio of open to 
closed porosity is mainly a function of the sintered density (they are approximate 
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equal in the level of density 7 g/cm; above 95% relative density, practically no 
open porosity exists) [3]. 
Depending on the type of pores, different parameters have been used to 
describe the dimensions of the pores. In the case of isolated and equiaxed pores, 
the metallographic evaluation gives fairly reliable results for the mean diameter 
and also the diameter distribution if the appropriate stereological corrections are 
applied. With interconnected pores, only the average channel diameter can be 
reliably given, since the pores, although twisted and irregular, are practically 
infinitely long. 
Pore shape is mentioned frequently as a parameter which influences 
mechanical parameters due to notch effects, although the load – bearing cross – 
section at a given porosity is larger in the case of spherical pores compared to 
angular one. The shape of pores is primarily a function of sintering temperature 
and time. Longer sintering at higher temperatures leads to progressive rounding 
of the pores due to the surface diffusion. When the shape factor is measured on 
metallographic sections, one must to take into account that pores connectivity 
plays a decisive role. In the case of isolated pores, a metallographic sections 
depicts predominantly the pore shape. In the case of interconnected pores, a 
metallographic study will incorrectly reveal the pore channels as more or less 
equiaxed features. 
The effective load bearing section is presented by many authors as a 
function of different materials’ characteristics. In this study the equation 1.1 
proposed by [3, 4] is considered. 
2
Φ=(1-K.ε) (eq. 1.1) 
p
K=(5.58÷5.7).f (eq. 1.2) 
pc
ε=1-(ρ/ρ) (eq. 1.3) 
th
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where f – pore profile parameter (0÷1); 
c
 ρ – material’s density; 
ρ – theoretical density. 
th
 The effect of density/porosity is a long time studied problem in PM. In 
pressed and sintered ferrous materials, the characteristic feature is the “integral” 
or “primary” porosity. This is in principle the porosity already present in the bulk 
powder, before compaction. Since the compaction of metal powders is commonly 
a cold working process, progressive work hardening occurs at the particle 
contacts and at some stage prevents further densification. Furthermore, the 
admixed organic lubricant that is inevitable in uniaxial die compaction, also 
absorbs considerable space in the compact and, being virtually incompressible, 
limits the densification of the metal skeleton. It remains an interconnecting 
network of pores, in part filled with lubricant, that is essential for the material 
during lubricant burn-out in the initial stages of sintering (lubricant in isolated 
pores would inevitably result in blistering). During the sintering process, the 
metal powder particles weld together at the pressing contacts through diffusion 
processes and form stable metallic bonds. The typical microstructure of a 
sintered steel as depicted by metallographic techniques is shown in figure 1.1 
[4]. The sections however can be misleading: during sintering the pore structure 
remains interconnected in most cases, i.e. at the common density levels of up to 
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7.4 g/cm for ferrous components (about 6% total porosity). Only through 
special pressing and/or sintering techniques or by powder forging density levels 
are attained where the interconnected pore network is dissolved into single 
isolated pores. Therefore, in most cases the sintered iron can be described 
through a “sponge” model, and only at rather high density levels the “swiss 
cheese” model, with isolated holes, applies. The structure of the pore network in 
sintered materials can be seen e.g. in resin replicas of the pore network, the iron 
skeleton having been etched away after resin impregnation [4]. The fact that the 
pores are mostly interconnected implies that the sintering contacts, which 
actually bear the load in the material (pores of course cannot be load-bearing), 
are isolated in these cases, and description of the microstructure has to focus at 
the sintering contacts. Furthermore, the pores form a very complex, 3-
dimensional structure that is virtually impossible to depict in a single 2-
dimensional metallographic section. Therefore, there should be a relationship 
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between the total area of the sintering contacts in the weakest cross section – 
which can be defined as the “load bearing cross section” – and the mechanical 
properties of the material. 
Figure 1.1: Scheme of the microstructure in PM steels [4] 
Therefore, it could be drawn that the effective load-bearing section of PM 
materials is a function of density and pores’ characteristics, figure 1.2. Both are 
strongly dependent on compaction method and sintering conditions. 
density 
green density sintering 
(compaction) condition 
pore 
structure 
load-bearing 
section 
Figure 1.2:Load-bearing section as a function of density and sintering 
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Compaction methods to obtain high green density 
In the field of PM, there are many methods to achieve high green densities. 
These methods include high velocity compaction [5], warm compaction [6], 
reduced and advanced lubricants [7, 8], and high compressibility powders [9]. 
Each of these methods has many advantages and disadvantages associated with 
it. Below, a brief description of the compaction methods used in this work is 
presented. 
The most widely used compacting technique involve die pressing in specially 
designed mechanical or hydraulic presses – Cold Compaction (CC). Densities up 
to 90 % of full solid density can be achieved by single pressing. The production 
rate usually varies from 5 up to 60 parts per minute. Cold compaction can be 
used to reach high densities by using elevated compaction pressures (>700MPa), 
high compressibility powders, or reducing lubricant level [5-8]. High compaction 
pressures have theoretical limits when considering part configuration and tooling 
requirements. The highest compressibility powders are typically pure iron, which 
is not always suitable for the very high performance applications [9]. Finally, 
reducing the lubricant level can be limited by the part configuration, since 
lubricant is required, especially during ejection [7, 8]. 
By elevating the powder and tooling temperature, Warm Compaction (WC) 
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can reach green densities in up to 0.15 g/cm greater than green densities 
achieved by conventional compaction [10]. Warm compaction also allows high 
green densities to be reached in virtually any part configuration. There are also 
advantages in warm compaction, such as, high green strength, and the 
possibility to achieve higher densities at lower compaction pressures [10]. 
Heating the powder and tooling can present an added level of complexity, 
however. 
High Velocity Compaction (HVC) is a powder compaction method that has 
many similarities to conventional compaction of PM parts. The most striking 
differences are that the stage of compaction can be 500-1000 times faster and 
that the ram speed of a HVC impact machine can be in the range of 2-30 m/s. 
Densification in HVC is achieved by intensive shockwaves created by a 
hydraulically-operated hammer, which transfers the compaction energy through 
the compaction tool to the powder. The mass of the hammer and its velocity at 
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the moment of impact determine the compaction energy and the degree of 
densification. An interesting feature of HVC is the possibility to perform multiple 
impacts [11]. In conventional compaction, the density does not increase 
significantly if the pressing sequence is repeated directly after the initial 
compaction. However, using HVC density can be increased impact by impact. An 
advantage of increased densification following multiple impacts, is that it is 
possible to compact large parts with moderately-sized equipment. Component 
production by HVC consists of the same processing steps as conventional 
compaction and the compaction tool design is similar to conventional compaction 
tools. Good production economy requires sufficient tool life and it is therefore 
reassuring that the endurance limit of HVC tools has been verified to exceed a 
minimum of 100,000 cycles in a full-scale compaction test [12, 13]. The 
mechanical properties of PM materials increase in proportion to the increased 
density achieved by HVC. A typical density increase of 0.3 g/cm³ has been 
recorded for high velocity compacted materials based on Distaloy AE and Astaloy 
CrM, compared to density levels representative of conventional compaction. As a 
result of the higher density, 20–25% higher tensile and yield strengths have 
been 
obtained [8, 9]. Radial springback of a compact is generally lower for HVC than 
for conventional compaction. Lower springback generally leads to the advantage 
of lower forces being required to eject a part from the die after 
compaction. HVC has the capability to produce parts with not only high densities, 
but also very consistent densities. Compaction tests on prototype gears have 
shown density variations of less than 0.01 g/cm³ [12]. 
Densities achievable with HVC combined with die wall lubrication (DWL), 
warm compaction (WC), and double press, double sintering (HVC-P2S2) are 
presented in Table 0.1. 
Table 0.1: Approximate densities achieved by HVC [8] 
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process density, g/cm
HVC 7.4 
HVC+DWL 7.6 
HVC+WC+DWL 7.7 
HVC-2P2S 7.8 
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Recent R&D in the area of High Velocity Compaction [13] concludes that the 
major advantages HVC can offer are: high density, the capability to compact 
very large parts, and the possibility to produce high numbers of parts in a short 
time. 
Processes that can offer improved material properties with no or moderate 
increase in manufacturing costs are of particular interest to the automotive 
industry, since they can expand the use of PM applications in 
cars and reduce the total cost for the end user. The approximate relative 
processing cost for various PM processes, using conventional single pressing as a 
reference, is presented in Figure 1.3. Production methods that only involve a few 
processing steps, have a high production capacity and require a minimum of 
investment for the equipment, are of special interest. This applies particularly if 
net shape or near net shape parts can be produced as a result of good tolerances 
and surface finish. A good cost/performance ratio explains why the conventional 
compaction method is the most established and most frequently used. This also 
explains the recent success of warm compaction, which is increasingly used and 
has now spread worldwide [13]. 
Figure 1.3: Relative processing cost for various PM processes [13] 
Figure 1.4 demonstrates the beneficial effect of HVC on pores’ structure and 
distribution. 
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Figure 1.4: Pores’ structure of Astaloy CrM: a) Cold compacted, 7.10 g/cm b) 
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High velocity compacted, 7.38 g/cmc) Double High velocity compacted, 7.58 
3
g/cm [14] 
To summarize the effect of density/compaction method on mechanical 
properties the scheme presented in figure 1.5 can be used. 
Figure 1.5: Mechanical properties of PM materials as a function of relative density 
[9] 
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Sintering 
In iron PM, sintering is most commonly carried out in continuous mesh-belt 
furnaces operating at 1100 to max.1150°C. Sintering temperatures of 1200 -
1350°C accelerate the homogenization of alloying elements and allow the use of 
beneficial but oxygen-sensitive alloying elements like chromium and manganese. 
With modern materials and furnaces, chromium alloys can now be sintered at 
1120°C.Mesh-belt furnaces cannot withstand temperatures above 1150°C. Time 
at temperature is usually not longer than 20 to 30 minutes, since longer sintering 
times yield only marginally improves properties which do not justify the 
increased sintering costs [9]. 
A sintering process are widely studied in the last 50 years [2, 3, 14 - 20]. In 
[20] Sanderow presents a scheme describing the general process (stages) when 
sintering. He divides the phenomenon occurring in two stages: “early stages of 
sintering” - conventional low temperature sintering, and “advanced stages of 
sintering” - high temperature sintering [20]. 
Early Stages of Sintering (Conventional <1150ºC) 
- HOMOGENIZATION – the as-cast, dendritic structure of the atomized 
particles is removed and microsegregation within the particles is 
eliminated; diffusion between powder particles begins to occur. 
- ALLOYING – as the diffusion process continues, admixed additives 
begin to form alloyed structures with the base ferrous particles. For 
species such as carbon this takes place early in the sintering process; 
for elements such as nickel and molybdenum diffusion is much slower 
and takes longer times and higher temperatures to achieve a 
reasonable level of homogeneity. 
- REMOVAL OF GASES/OXIDES – chemical reaction between the sintering 
atmosphere or admixed additives such as graphite and the surface 
oxides on the metal particles also begins early in the sintering cycle. 
This breakdown of oxides and removal of adsorbed gases cleanses the 
metal particle surfaces and promotes the diffusion process. 
- PARTICLE BONDING – the formation of solid bridges or necks between 
individual or clusters of powder particles is the critical results of the 
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early stages of sintering. These particle bonds give the powder mass 
integrity and mechanical strength, figure 1.6. 
Figure 1.6: Particle bonding [20] 
Advanced Stages of Sintering (High Temperature >1200ºC) 
- DENSIFICATION – as the sintering process continues at higher 
temperatures the inherent in the powder mass is reduced as pores are 
eliminated by bulk diffusion to grain boundaries. This reduction in the 
amount of the porosity results in an increase in the density of the 
powder compact, figure 1.7-I. 
- POROSITY SHAPE – the remaining pores in the PM structure lose their 
angular, irregular nature and becomes smooth, tending toward perfect 
spheres, as the sintering temperature increases, figure 1.7-II. 
- GRAIN GROWTH – the individual powder particles lose their identity 
completely as grain boundaries move across prior particle boundaries. 
Larger grains replace the original fine particle structure, figure 1.7-III. 
Figure 1.7: Stages of solid state sintering [20] 
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- LIQUID PHASE – depending on the chemical constituents in the powder 
mass and the sintering temperature, a transient or permanent liquid 
phase may be formed. This liquid phase will accelerate particle 
rearrangement and diffusion, thereby aiding densification and pore 
elimination. For some additives, such as copper and phosphorus, liquid 
phase sintering will occur at conventional temperatures, while for 
silicon iron and the tool steels high temperature sintering is required. 
It should be realized that the phenomena attributed to the early stages of 
sintering will continue during high temperature sintering. Thus a greater number 
and more compete particle bonding, as well as more homogenous alloying, will 
occur during the advanced stage of sintering than found during the early stages. 
After sintering the temperature of the powder mass must be lowered to room 
temperature under controlled atmosphere cooling. Depending on the rate of 
cooling trough the critical steel transformation temperature range, the resultant 
structure may be annealed, normalized or quenched [20]. 
Besides temperature and cooling rate, another important factor of the 
sintering process is the atmosphere. The main purpose of sintering atmospheres 
is to protect the powder compacts from oxidation during sintering and to reduce 
residual surface oxides in order to improve the metallic contact between adjacent 
powder particles. A further purpose of sintering atmospheres is to protect 
carbon-containing compacts from decarburization [9]. 
Pure hydrogen, electrolytically or cryogenically produced, is the most 
unproblematic atmosphere for sintering carbon-free iron powder parts. As a rule, 
however, it is not economical, except in combination with high priced products 
such as alnico magnets and stainless steel parts. An excellent substitute for pure 
hydrogen are the atmospheres, which consists of 75-95% H and 5-25% N. The 
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strong reducing action of this gas mixture is favourable in eliminating residual 
oxides which are present in all commercial iron powders. It is easy to handle 
and, although it is not the most economic atmosphere, it eliminates many 
production problems and yields a uniform and high quality sintered product [9]. 
The most widely used atmospheres primarily because of their lower cost, 
are produced by partial combustion of hydro-carbons. By variation of the air-to-
gas ratio, a wide range of compositions is obtained. For practical applications, 
since the combusted gas contains water vapour it must be dried to a dew point 
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of less than 0ºC for satisfactory operation with iron components. Hydrocarbon 
gas, such as methane, butane or propane, reacted with a limited amount of air 
may contain up to 45% of hydrogen, some carbon monoxide and dioxide with 
nitrogen as the remainder. Because of the endothermic nature of this reaction, 
external heat has to be supplied, and for that reason the resulting atmosphere is 
called endogas. 
Alloying Elements 
The powder metallurgical technique, when including only cold pressing of a 
plain iron powder in a closed die and subsequent sintering of the green compact, 
results in parts with a porosity in the range 10-20%. Typically, the tensile 
strength of such a material is 150-200 MPa with an elongation of 8-12 %. By 
application of a more costly double pressing technique, i.e. a process where the 
density of the sintered compact is increased substantially in a second pressing 
operation (coining), the porosity can be decreased to about 8 %. The material 
then shows a tensile strength of 300-350 MPa, with a low elongation of 2-4%. By 
performing a second sintering operation the elongation can be increased to 20-
25 % and, simultaneously, the tensile strength drops to 250-300 MPa [21]. 
In ’70 of the last century in Europe, this double pressing and double-
sintering technique was the one originally chosen to improve the mechanical 
properties of sintered iron by moving its density towards that of the wrought 
materials which it was to replace. The improvements thus obtained were of 
importance in the early days of ferrous powder metallurgy, but were not of such 
a magnitude that plain sintered iron could be utilized in structural components 
subjected to any higher stresses. It should also be pointed out that the iron 
powders which were available at that time had a rather poor compressibility 
except for the expensive electrolytic powder, which meant that extreme 
pressures had to be applied both in powder compacting and in coining in order to 
get the desired high density levels [21]. 
It became obvious that, in order to achieve technically significant 
improvement of the mechanical properties of ferrous PM materials, the use of 
alloying additions to the iron was necessary. The development of suitable 
alloying systems began early. The nature of the powder metallurgy process, in 
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many respects, offered unique possibilities of introducing alloying elements, but 
it also implied considerable problems in controlling the behaviour and effects of 
the alloying elements in the various stages of the process in order to arrive at 
the expected physical and mechanical properties of the finished product. 
Alloying additions, in order to improve the mechanical properties of the 
sintered material, must therefore be of such a kind and made in such a manner 
that this dimensional precision is not lost. The factors of importance in this 
respect are segregation phenomena during the handling of the powder and 
dimensional variations occurring during sintering. 
Alloying elements are used in PM steels to promote hardenability and 
increase the mechanical strength of structural parts. These elements can be 
added: by admixing (blending) elemental alloying powders to the ferrous 
powder, by fully dissolving the alloying elements into the liquid steel prior to the 
powder manufacturing process (prealloyed powders), or by partially alloying 
between two or more base powders (diffusion bonding). In few cases also 
coating the particles of an iron powder with an alloying metal by electrochemical 
or other metal deposition methods is used. The alloying techniques has a large 
impact on the microstructure and, consequently, on the sintered and heat 
treated mechanical properties of PM parts [21]. 
Each technique (figure 1.8) has its advantages and disadvantages. 
Figure 1.8: Schematic presentation of the alloying methods used in PM [21] 
High compressibility can be maintained by blending a pure base iron powder 
with powders of alloying elements, but there is a risk of segregation, which may 
cause dimensional scatter of the sintered components. 
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