EFFECTS OF THERMAL DEBINDING ON BINDER REMOVAL OF
STAINLESS STEEL AISI 420L FEEDSTOCK
Escobar, C. F.1, Martins, V.2, Trajano, W. T.1, Schaeffer, L.2, Santos, L. A.1
1
Departamento de Engenharia de Materiais, Universidade Federal do Rio Grande do Sul, Porto Alegre (RS),
Brasil.
2
Departamento de Engenharia Metalúrgica, Universidade Federal do Rio Grande do Sul, Porto Alegre (RS),
Brasil
E-mail: camila.escobar@ufrgs.br
Abstract. Metal injection molding (MIM) process is a technology that allows the fabrication of metallic parts
with high shape complexity, high final density when compared to conventional powder metallurgy and it’s
applied for small components, such as stainless steel surgical instruments. This process involves four steps:
mixing metal powder with the polymer system (binder) to form feedstock, injection of the feedstock, removal of
the binder (debinding) and sintering. The binder removal is the most critical step of the process because it
involves the removal of the binder used to inject the piece without compromising the shape integrity. The binder
is primarily formed by polymers, and consists of a temporary vehicle which compress the metallic particles in
the mold and promote fluidity during the injection process. In the present work was investigated the thermal
debinding of feedstock CATAMOLD® 420 A. This was composed of the powder AISI 420 stainless steel and
polyacetal (POM - polyoxymethylene) as binder and is widely used in the manufacture of surgical instruments. It
was studied the different heating rates for thermal debinding. Weight loss, density, porosity, microstructure and
shrinkage were measured. Differences in the efficiency of the binder removal by thermal debinding were found.
Palavras-chave: Metal injection molding (MIM), Stainless Steel AISI 420, Surgical
instruments, thermal debinding.
1.
INTRODUCTION
Metal parts with complex geometry and high dimensional accuracy are obtained by the
metal injection molding process (MIM). This processing technique for metal powders has
been developed in the USA in the early 1970 [German, 1997]. It’s a powder metallurgy
process which combines the possibility of producing metal parts with complex geometry and
the easiness and productivity of injection molding process. This gives high dimensional
accuracy to the final piece, the production of components in small sizes and high geometric
complexity, defects surface free, high density of sintered part, obtaining components with
isotropic microstructure, high productivity and process stability.
Metallic materials have good mechanical properties, for this reason are frequently used
in implants that undergo severe mechanical stress and surgical instruments, and metals from
the stainless steels are the most used for this purpose, because of its good biocompatibility
properties, excellent mechanical strength, relatively low cost and corrosion resistance.
Martensitic stainless steel has good properties necessary for surgical instruments, like wear
and corrosion resistance [Perot et al, 2003].
The binder removal process is the most critical MIM step, because it consists entirely
eliminate binder without causing damage, such as bubble, crack, slump and deformation,
resulting in the damage of the mechanical properties and dimension precision of final parts
[Li et al, 2003; Zaky, 2004]. Catamold ®420A is a feedstock developed from BASF®. It`s is
based on a polyoxymethylene (POM) or polyacetal binder, a semi-crystalline thermoplastic
material with good processing characteristics, high dimensional stability, high rigidity and
good warm strength [Bloemacher and Weinand, 1997]. The main debinding method used for
POM removal is catalytic debinding, however this requires specific debinding furnace and
gaseous nitric acid (> 98,5 %), which makes the process more expensive than thermal
debinding, that can be realized in the same furnace of sintering process and it`s not required
gaseous acids, likewise the condensation of nitric acid must be strongly considered as a
corrosion problem for the thermal treatment of the metallic product. Moreover the residue
amount generate in the catalytic debinding is greater than thermal debinding.
In other work it was investigated the cause of defects in large ceramics parts obtained
from injection molding of Catamold AO-F, the POM was removed by catalytic debinding
[Krug et al, 2001]. According the authors, the defects are generate in the catalytic debinding
step due changes or interruptions in the nitric acid rate supply, interruptions to the furnace gas
circulation or temperature changes in the furnace environment.
In this work, we studied the influence of thermal debinding in the specimen’s
properties, and the morphology and characteristics of the specimens were evaluated.
2.
MATERIALS AND METHODS
2.1
Materials
Commercially available Catamold® 420 A feedstock (BASF, Germany) was used as
material for samples injected. This feedstock was based on a polyoxymethylene (POM)
binder. Table 1 gives the chemical composition of the stainless steel AISI 420A component of
Catamold ®420A. Figure 1 show the scanning electron microscopy (SEM - Hitachi TM-3000)
image of this feedstock . .
Table 1
Chemical composition of AISI 420A stainless steel (wt%).
C
Cr
Mn
Si
Fe
0.18 – 0.30
12.0 – 14.0
≤ 1.00
≤ 1.00
Balance
Figure 1: SEM of feedstock.
2.2
Metal injection molding process
The samples were injected on a machine Thermo Scientific Haake Minijet II. The injection
parameters used are listed in the Table 2.
Table 2. Injection moulding settings.
Injection pressure (bar)
Injection time (s)
Holding pressure (bar)
Holding time (s)
Barrel temperature (◦C)
Mould temperature (◦C)
500
6
350
3
210
120
In this study, the rate of thermal binder extraction was 2, 1, 0.5 or 0.2°C/min until the
temperature of 475 °C for 20min, after that, it was heated, at rate of 10 °C/min, until the
temperature of 1150°C for 60min. The polymer debinding and sintering curve is illustrated in
figure 2. The furnace used for heating process was a vacuum furnace.
Figure 2: Debinding and sintering process curve. The debinding rate were x= 2, 1, 0.5 or 0.2°C/min.
2.3
Characterization techniques
Differential scanning calorimetry (DSC) was done on a Universal TA Instruments for
analysis of Catamold ®420A feedstock. Samples of about 10 mg were heated with a linear
heating rate of 10°C/min in the temperature range of 30-1000°C. The thermogravimetric
measurements were performed using a TGA Q50 (TA Instruments) under nitrogen
atmosphere. The granulated sample (5 mg) were heated from ambient temperature to
approximately 1000°C at rate of 10 °C/min. Samples for microstructure analysis were
prepared according ASTM E 3-95 – “Standard Practice for Preparation of Metallographic
Specimens” and etched with Vilela’s reagent (1 g picric acid, 5 ml HCl and 100 ml ethanol).
The corresponding microstructures were examined by the usual optical microscopy technique
in Zeiss Lab.1 reflected light microscopy. Analysis of X-ray diffraction (XRD) were made to
identify the phases in the samples, following the recommendations of the ASTM E 975-95,
operating with radiation of Kα Cu 50kV and 100mA, scanning angle (2θ) in the range 25120°. Flexural strength was evaluated on Instron Universal testing machine and was
supported on three ball bearings equally spaced around the periphery. The load was applied to
the center of the specimen at speed of 0.5 mm per minute.
3.
RESULTS
The DSC and TGA/dTG curves are illustrated in figure 3. In the DSC curve can be
observed the heat flow peak at about 164.87°C in heating curve, indicating the melting point
of the binder (Tm). The crystallization temperature, on cooling step, was 148.86°C. Molding
must be done at a temperature higher than the melting point of the binder, that is above
164.87°C. On the other hand, the mold temperature must be below 148.86°C to improve the
mechanical property of injected material. The thermal decomposition behavior of feedstock
was investigated by TGA/dTG curves. Polymer decomposition start at 384.59°C and ends at
477.75°C, after this temperature the mass of sample increases, related to the oxidation of
steel. The thermal binder removal requires low polymer debinding rate, therefore the final
binder removal temperature must be lower than 477.75°C, in order to avoid metal oxidation.
Fig. 3: DSC and TGA/dGT feedstock curves.
The phases of the specimens were identified using XRD and micrographs. The
diffractograms is showed in figure 4. Carbides were observed in all samples. The M23C6
carbides amount decrease as thermal heating rate decreases. It was found FeMn4 and CrMn3
intermetallic compounds in the samples with the lowest rate. At high binder removal rates
austenite phase is found, this can be related with carbides amount in the steel. At 2 and
1°C/min, most frequently, the carbon content of the steel composition is present in the
carbides, making the surrounding area poor in C and not allowing the martensitic formation in
this regions. There are similar pattern with peaks of martensite in all diffractograms.
Figure 4: XRD diffractograms.
The micrographics of sintered samples showed a porous structure with pores of
different sizes. The figure 5 shows microstructure of the sample with thermal extraction rate 2
ºC/min. Spherical carbides contained in the metallic matrix are identified after etching with
Vilela´s reagent. Large martensite amount can be viewed in figure 4, that can ber related to
the results of X-ray diffraction (fig. 4). The Figure 6 shows a microstructure of the sintered
sample with thermal extraction of 1°C/min. The similar globular carbides are present in both
2 and 1°C/min specimens, and porous structure.
Figure 5: Specimen with binder thermal extraction of 2°C/min, a) without etching and b)
Vilela`s reagent etching.
Figure 6: Sample with thermal extraction of 1°C/min, a) without etching and b)
Vilela`s reagent etching.
The Figure 7 shows, in sample with heating rate of 1oC/min , the presence of fine
carbide particles and a slightly defined grain boundary. Note that there was a porosity
decrease due to the lower rate, allowing exit of gas. Analyzing the metallographic and X-ray
diffraction data (fig. 4) we can conclude that the carbides are embedded within the matrix
martensitic. In the figure 7b can see diffusion between carbides and the growth of grain
boundary, maintained the martensitic structure in the matrix with the inner carbides. The
sample with thermal extraction rate of 0.2°C/min is showed in figure 8. It can be observed the
homogeneous distribution of carbides and others structures, maybe related to the intermetallic
compounds founded in the X-ray diffractogram (fig. 4).
Figure 7: Sample with thermal extraction of 0.5°C/min, a) without etching and b)
Vilela`s reagent etching. RETIRAR A SETA DA FIGURA
Figure 8: Sample with thermal extraction of 0.2°C/min, a) without etching and b)
Vilela`s reagent etching.
The sintered specimens obtained after polymer debinding rate of 2, 1, 0.5 and 0.2°C/min are
illustrated in figure 9a. Bubbles are found in the specimens with removal polymer rate 2, 1
and 0.5 °C/min. The bubbles amount decrease with decrease of polymer debinding rate. This
behavior occurs due the high gas formation rates on internal body and diffusion of volatile
constituents to the surface of the green body, where they evaporate [Lewis, 1997; Calvert and
Cima, 1990]. At binder removal rate of 0.2°C/min was not found bubbles or other damages
frequently associated to binder removal process. The density of sintered samples, figure 4c,
increases with decrease of polymer debinding rate, only isn’t observed this behavior in
specimen binder burned at 0.2°C/min.
Fig. 9: a) Sintered samples and b) density curve.
The flexural strength of sintered PIM samples are show in figure 10. The flexural strength that
samples are also influenced by the binder removal rate. The biggest value is found for
0.5°C/min specimen. The sample strength increases according with decrease polymer
debinding rate because of defects decrease (pores and bubbles). In general, the strength values
measured are very low for sintered martensitic stainless steel [Klar and Samal, 2007]. These
values can be related to presence of many pores in the microstructure and coarse carbides.
The presence of carbides in the steel microstructure has a decisive effect on their properties,
brittleness increases and corrosion resistance decreases. The effects on the corrosion
resistance occur due decreases of chrome amount in the metallic matrix, mainly for formation
of Cr23C6 carbides [Andrés et al, 1998].
Fig. 10: Flexural strength of sintered specimens.
4.
CONCLUSIONS
The thermal extraction of the binder contained in Catamold® 420 feedstock greatly
influences the microstructure, the final geometry, density and surface finish. It was showed
that the sample must be cooled rapidly, after sintering to prevent the formation of carbides.
Thermal extraction rate of 0.2 ºC/min is more appropriate for the removal of the polymer in
Catamold 420. Thermal extraction rate above 0.5 ºC/min causes increase on porosity, surface
defects and warping.
ACKNOWLEDGEMENTS
The authors acknowledge financial support from CNPq and Capes, LdTM, Labiomat and
LACER for contribution in the developed this work.
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