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

1Departamento de Engenharia de Materiais, Universidade Federal do Rio Grande do Sul, Porto Alegre (RS), Brasil.

2Departamento 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 1Chemical composition of AISI 420A stainless steel (wt%).

C Cr Mn Si Fe0.18 – 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) 500Injection time (s) 6Holding pressure (bar) 350Holding time (s) 3Barrel temperature (◦C) 210Mould temperature (◦C) 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 25-120°. 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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