EXPERIMENTAL INVESTIGATION OF WELD JOINTS BETWEEN SINTERED Nb MODIFIED HK30 STAINLESS STEEL AND WROUGHT/CAST STAINLESS STEELS

Very important property of powder metallurgy parts is ability to join to components produced by different manufacturing technologies or dissimilar materials. Properties of powder metallurgy Nb modified HK30 components are highly influenced by conditions applied during sintering. Weldability of sintered components can be improved using favorable sintering conditions. In this regard, effect of sintering parameters on fusion weldability of Nb modified HK30 is presented in this paper. Investigation of weld joints between HK30, produced by different sintering conditions, and cast HK30 stainless steel is performed. In addition, examination of welds between sintered HK30 and wrought 304 stainless steel is also performed. Microstructural examination and hardness testing of fusion zones and heat affected zones were done for different combinations of base material. steel sintered in nitrogen atmosphere at temperature of 1310 ⁰ C and cast steel. Average hardness of cast steel was 282 HV, while fusion zone and sintered steel were 288 HV and 283 HV, respectively. Slightly refined structure of fusion zone was observed when filler material is used. Differences in thermal conductivity of the porous sintered and cast component affected the heat distribution during welding which caused differences in fusion zone geometries. Geometry of fusion zone is extended to sintered side of the weld.

Very important property of powder metallurgy parts is ability to join to components produced by different manufacturing technologies or dissimilar materials. Properties of powder metallurgy Nb modified HK30 components are highly influenced by conditions applied during sintering. Weldability of sintered components can be improved using favorable sintering conditions. In this regard, effect of sintering parameters on fusion weldability of Nb modified HK30 is presented in this paper. Investigation of weld joints between HK30, produced by different sintering conditions, and cast HK30 stainless steel is performed. In addition, examination of welds between sintered HK30 and wrought 304 stainless steel is also performed. Microstructural examination and hardness testing of fusion zones and heat affected zones were done for different combinations of base material.

…………………………………………………………………………………………………….... Introduction:-
Unique properties of parts produced by powder metallurgy caused expansion of this manufacturing technology in production of parts for various industrial applications. Some of advantages of powder metallurgy technologies compared to other manufacturing technologies are: efficient material utilization, near net shape production, ability to process very demanding materials, easier achievement of desired microstructure, production of porous components etc. One of the very important factor for some application of PM parts is ability to join to components from dissimilar materials, produced by powder metallurgy or other manufacturing technologies. Residual porosity and reduced thermal conductivity of sintered components affect heat transfer during welding. In addition to the effect on physical properties of material, residual porosity can contain entrapped gases which can adversely affect weldability of the PM components. Austenitic Nb modified stainless steel HK 30 sintered in N 2 atmosphere can contain more than 0.5 % [1] of nitrogen, which is released during fusion welding and causes porosity in fusion zone (Figure 1.). Application of sinter joining process on this steel resulted in the weld joint with characteristics almost ident ical to the base material, because the melting is avoided [2]. On the other side, nitrogen increases hardness and strength of the steel [1,3]. Grain structure of sintered base material has also an impact on solidification behavior of fusion zone during welding. Sensitivity of fusion zone to hot cracking depends on its structure, whereas fine grains of fusion zone are more resistant than coarse columnar structure [4].  Sintering in hydrogen and argon results in more ductile material, lower hardness and strength, lower percent of porosity and reduction of carbon and nitrogen percent [1,3]. If material has high amount of porosity with reduced contact area between particles, thermal stresses in heat affected zone can cause cracking [5]. Residual porosity, carbon percent, nitrogen percent and grain structure of sintered components are factors which significantly depend on sintering parameters. Thus, sintering parameters must be taken into account during weldability analysis of sintered components. In this regard, investigation of weld joints between sintered Nb modified HK 30 stainless steel (sintered steel in following text) and wrought/cast stainless steels for different sintering conditions were performed in this work. Experimental probes were done using laser and plasma welding processes.

Experimental work
Metal injection molding technology was used for production of sintered components (Tabele 1.). Sintering was done in three different atmospheres and two levels of temperature. Primary debinding and residual binder debinding were performed by catalytic debinding and thermal debinding processes, respectively. Sintering was done in hydrogen, nitrogen and argon atmosphere at temperatures of 1200 ⁰C and 1310 ⁰C. First part of experimental work refers to examination of fusion zone, partially melted zone and heat affected zone of laser weld joint between sintered and cast components. In addition, investigation of plasma weld joint between component sintered in dissociated ammonia and wrought 304 austenitic stainless steel was performed. Cooling rate of melt decreases as distance from fusion line increase, which causes different morphology of fusion zone (Figure 2. a, b). The lack of material in the fusion zone may be the result of the wicking of the melt by the porous base material. This problem is particularly pronounced as the percentage of porosity in the base material increases. Average hardness of steel sintered in argon and temperature of 1200 ⁰C is 165 HV. Sharp increase in hardness is observed in the fusion zone. Increase of density, as a consequence of melting, contributed to hardness increase to an average value of 250 HV (Figure 3.). Parts sintered in nitrogen atmosphere at temperature 1200 ⁰C showed average hardness of 220 HV, while the average hardness of the fusion zone is 230 HV (Figure 3.). Strengthening of material due to nitrogen absorption from sintering atmosphere resulted in higher hardness. The release of nitrogen during welding causes the formation of very porous fusion zone, weak weld and appearance of cracks (Figure 1.). Nitrogen in sintered steel may be in form of solid solution, nitrides and entrapped in pores [6]. The best weldability was achieved during welding of parts sintered in hydrogen atmosphere. Fusion zone and heat affected zone are almost without porosity for both 1310 ⁰C and 1200 ⁰C cases. Hardness across the plasma weld joint of steel sintered in H 2 and temperature of 1310 ⁰C, varied from 175 HV in base material to 196 HV in fusion zone. However, reduced mechanical properties of steel sintered in hydrogen atmosphere may be the limiting factor for some applications of this steel. Significant hardness variations are not observed across the weld joint between steel sintered in nitrogen atmosphere at temperature of 1310 ⁰C and cast steel. Average hardness of cast steel was 282 HV, while fusion zone and sintered steel were 288 HV and 283 HV, respectively. Slightly refined structure of fusion zone was observed when filler material is used. Differences in thermal conductivity of the porous sintered and cast component affected the heat distribution during welding which caused differences in fusion zone geometries. Geometry of fusion zone is extended to sintered side of the weld. a) b) Figure 6:-Microstructure of plasma welded joint between sintered HK30 steel and 304 austenitic stainless steel: a) weld joint b) fusion zone.
As seen before, very good weldability was achieved using hydrogen as sintering atmosphere, even in case when very low density parts were welded. Large hardness variations trough weld joint between steel sintered at 1200 ⁰C (H 2 ) and cast steel was observed. Hardness varied from 125 HV in sintered side up to 280 HV in fusion zone and cast base material. Low density of sintered part and reduced carbon percents are responsible for reduced hardness in sintered side. Taking into account poor mechanical properties of parts sintered hydrogen and argon and low weldability of N 2 sintered parts, mixture of H 2 -N 2 atmosphere can be used in order to fulfill the most requirements.
In this regard, plasma welding of HK30 steel sintered in mixture of H 2 -N 2 to wrought 304 austenitic stainless steel is investigated (Figure 6.). Fusion zone and heat affected zone are without porosity and cracks. Hardness of 304 base material is 196 HV, while average hardness across the fusion line and fusion zone are 215 HV and 219 HV, respectively. Average hardness of heat affected zone of sintered base material is 180 HV.

Conclusion:-
Properties of weld joints between sintered HK 30 stainless steel and wrought/cast stainless steels are very dependent on parameters applied during sintering. Residual porosity, absorbed gases and grain structure are factors which have high influence on weldability of sintered component. Also, grain structure of sintered base material affects solidification process of fusion zone. Refinement of fusion zone structure is observed after welding of sintered components with finer grain structure. Differences in thermal conductivity of the porous sintered components and cast component caused differences in fusion zone geometries.
Sintering in nitrogen atmosphere resulted in improved mechanical properties of sintered steel, but reduced weldability of steel. Lower sintering temperature and high percent of porosity for parts sintered in N 2 showed very porous fusion zone. Porosity in fusion zone appears due to nitrogen released from steel during welding. Steel sintered in hydrogen and argon atmosphere showed very good weldability, but reduced mechanical properties can limits the application of this steel for some operation conditions. Porosity free and crack free weld joint between HK30 steel sintered in H 2 -N 2 atmosphere and 304 stainless steel are observed.