Effect of Cu3P addition on sintering behaviour of elemental powders in the composition of 465 stainless steel

Abstract

The addition of Cu₃P for developing the high strength 465 maraging stainless steel from elemental powders was studied. The sintering parameters investigated were sintering temperature, sintering time and wt-%Cu₃P. In vacuum sintering, effective sintering took place between 1300 and 1350°C. The maximum sintered density of 7·44 g cm^?3 was achieved at 1350°C for 60 min with 4–6 wt-%Cu₃P. More than 6 wt-%Cu₃P content and temperature >1350°C caused slumping of the specimens. The sintered specimens were heat treated and a maximum ultimate tensile strength (UTS) of 767 MPa was achieved with 4 wt-%Cu₃P content. The maximum hardness of 45·5 HRC was achieved in heat treated condition with 4 wt-%Cu₃P content. Above 4 wt-%Cu₃P content increase in density was observed whereas the response to heat treatment decreased. Fracture morphologies of the sintered specimens were also reported. A comparison of sintering behaviour and mechanical properties of elemental powders with prealloyed powders was also given in the present study.     

Liquid phase sintering of ferrous powder by carbon and phosphorus control

Abstract

Powder mixtures composed of liquid forming master alloy powder and coarse iron powder were sintered to near full density by having a high amount (20 wt-%) of liquid phase during sintering. This was made possible by the use of the Fe-P-C system with or without Cu. Without post-sintering treatment, a brittle microstructure was obtained. By means of altered C and P control and decarburisation heat treatment of the as sintered material, the final non-brittle microstructure was achieved. Using the open porosity and liquid phase as a diffusion path, rapid decarburisation is created and the local combination of carbon and phosphorus in the microstructure is avoided. In this way, iron phosphide is not formed on grain and/or particle boundaries. Presence of pores is confirmed to be beneficial for grain growth control.     

Role of reinforcement in sintering of SiC/316L stainless steel composite

Abstract

Austenitic stainless steels with improved corrosion resistance are gaining wide popularity. However, their applications are limited because of their poor tribological properties. The present work was undertaken to improve the overall performance of 316L stainless steel by reinforcing it with SiC. During the processing of the 316L SS composite, the 316L SS matrix was found to interact strongly with the SiC at 1100°C resulting in the formation of low melting Fe–SiC phase. An attempt to process SiC/316L SS composite above this temperature resulted in complete melting of the composite compact.     

Consolidation and green body characteristics of gelcast metallic powder

Abstract

A preliminary study concerning the compaction of metallic powder was carried out in order to investigate a rarely explored route in powder metallurgy, to form complex geometry parts, known as gelcasting. Green bodies produced with as supplied stainless steel powder showed a tendency to form foam, which affected the surface finishing. The mechanical behaviour of green compacts was also affected by the processing additives present in the metallic powder. Organics in the as supplied powder were removed by thermal treatment at 500°C and additional samples were produced. Although no difference in green density was observed, these samples displayed better surface finish and mechanical characteristics, as a result of improved adhesion between the polymer network and particle surface. The results showed that the gelcasting process is able to produce green parts suitable for subsequent thermal treatment.     

Surface self-densification in boron alloyed austenitic stainless steel and its effect on corrosion and impact resistance

Abstract

A boron alloyed AISI 316L, sintered in pure hydrogen at 1250°C, was investigated to study the effect of the microstructure, with particular reference to the near full dense and boride free surface layers, on corrosion resistance and impact properties. The near full dense and homogeneous austenitic surface layer results from the flow of the liquid towards the bulk of the specimens during sintering. It significantly improves the corrosion resistance, while impact properties are strongly influenced by the bulk microstructure, where the boride network creates favourable conditions for both nucleation and propagation of crack.     

Effect of atomisation medium on sintering properties of austenitic stainless steel by eliminating influence of particle shape and particle size

Abstract

Gas and water atomised 316L stainless steel powders with similar powder morphology and particle size were injection moulded and sintered. The results show that compacts prepared from the gas atomised powder exhibit higher density and tensile strength, whereas those prepared from the water atomised powder exhibit higher elongation, finer grain size and superior corrosion resistance. Chemical analysis shows that the water atomised powder has a higher Si and O content, and microstructural analysis of the sintered compacts reveals that SiO₂ particles disperse as a second phase in the compacts prepared from the atomised powder, which accounts for the property behaviour. Due to the presence of SiO₂, the porosity increases, whereas the pore coarsening and grain growth are inhibited. Besides, SiO₂ particles can also improve the passivation effect of stainless steel, and hence increase the corrosion resistance.     

Numerical simulation of metal powder die compaction with special consideration of cracking

Abstract

Powder die compaction is modelled using the finite element method and a phenomenological material model. The Drucker–Prager cap model is modified with the goal to describe the formation of cracks during powder transfer, compaction, unloading, and ejection of the parts from the die. This is achieved by considering the cohesive strength and the cohesion slope, which characterise the current strength of the powder compact in the Drucker–Prager model, as state dependent variables. Evolution equations are formulated for these variables, so that the strength increases by densification and decreases by forced shear deformation. Some of the parameters appearing in the evolution equations are determined from measured green strength values. An iron based powder (Distaloy AE) is used for the experiments. Examples are shown to demonstrate that the density distribution can be calculated accurately as compared with an experiment, that cracking can be modelled at least qualitatively correctly, and that the compaction of complex 3D parts can be simulated.     

Microstructural and mechanical properties of injection moulded gas and water atomised 17-4 PH stainless steel powder

Abstract

This paper describes the microstructural and mechanical properties of injection moulded 17-4 PH stainless steel gas and water atomised powder. Gas and water atomised stainless steel powders were injection moulded with wax based binder. The critical powder loading for injection moulding were 62·5 and 55 vol.-% for gas and water atomised powders respectively. Binder debinding was performed using solvent and thermal method. After dedinding the samples were sintered at different temperatures for 1 h in pure H₂. Metallographic studies were conducted to determine to extend densification and the corresponding microstructural changes. The results show that gas atomised powder could be sintered to a maximum (98·7%) of theoretical density, and water atomised powder could be sintered to a maximum (97·08%) of theoretical density. Maximum tensile strength was obtained for gas atomised powder sintered at 1350°C. The tensile strength of the water atomised powder sintered at the same temperature was lower owing to higher porosity. Finally, mechanical tests show that the water atomised powder has lower mechanical properties than gas atomised powder.     

Microstructural evolution of austenite under conditions simulating thin slab casting and hot direct rolling

Abstract

The hot direct rolling (HDR) of thin slabs introduces some new microstructural phenomena with respect to conventional hot rolling of steels. This paper aims to investigate the microstructural changes of as cast austenite under these conditions. Current laboratory techniques for HDR simulation require a freshly cast slab for every experiment and a perfect link between casting and hot deformation. The present work adopted a new approach; the C–Mn steel is substituted by austenitic Fe–30Ni alloy, Conventional reheating before rolling replaces the direct link. The experimental ingot casting of Fe–30Ni alloy resulted in a solidification structure in good agreement with that of thin slabs of C–Mn steels. From metallographic observations, a mixed softening process and a strong grain refinement and homogenisation characterise the microstructural changes during HDR simulation. The microstructural behaviour and the grain refinement measured for the Fe–30Ni alloy is closely comparable with that predicted for C–Mn steels for the same conditions. The steel substitution appears to constitute a suitable and advantageous experimental approach for HDR simulation.     

Effect of MnS powder additions and vibratory ball peening on corrosion resistance of sintered 316LSC stainless steels

Abstract

As sintered parts are to be machined after sintering, the MnS powder is usually added to improve the machinability. Vibratory ball peening is used for deburring and improving the surface finish of sintered components in local PM industries. The effect of the MnS powder content and vibratory ball peening on the corrosion resistance of the sintered 316LSC alloys was investigated. Experimental results show that the addition of MnS powder slightly decreases the sintered density. The weight loss rate of the sintered specimens immersed in the 10%FeCl₃ corrosion test solution increases slightly with increas- ing MnS content, but decreases with increasing sintering temperature. Vibratory ball peening effec- tively decreases the weight loss rate of the sintered stainless steels. The chromium atoms actively migrate across the phase boundary and diffuse into the MnS particles during sintering. This intensive chromium diffusion affects the corrosion performance of the sintered alloys with MnS added. The surface morphology of the as sintered and the ball peened specimens before and after the corrosion test were studied with a SEM.     

Effect of tool coatings on ejection characteristics of iron powder compacts

Abstract

Powder metallurgy (PM) part makers heavily rely on part density as a mean of controlling part performance. Higher compaction pressures may be used to obtain higher densities and better properties. However,ejection stresses usually increases with compacting pressure. Those stresses may affect significantly part quality (surface finish, formation of cracks and lamination) and tool wear.

Different methods may be used to minimise ejection stresses, such as the use of admixed lubricant, die wall lubrication and the modification of tool surfaces. This paper presents an approach to evaluate the effect of tool coatings on the ejection of ferrous compacts. The method consists of evaluating the ejection characteristics of core rods with different coatings. The results obtained show that ejection characteristics are sensitive to tool coatings. Coating the surface of the core rods yields important variations of the stripping pressure (2×) and ejection energy (1·6×). No clear correlations between the ejection characteristics and the part surface finish were observed.     

Novel hafnium containing steels for power generation

Abstract

There is clear evidence that creep damage in power plant steels is associated with grain boundary precipitates. These particles provide favourable nucleation sites for grain boundary cavities and microcracks. The formation of M₂₃C₆ carbides as grain boundary precipitates can also lead to grain boundary chromium depleted zones which are susceptible to corrosive attack. Such precipitates are the causing loss of creep life in the later stages of creep because of their very high coarsening rate. Through Monte Carlo based grain boundary precipitation kinetics models, combined with continuum creep damage modelling it is predicted that improvements in creep behaviour of power plant steels can be achieved by increasing the proportion of MX type particles. Studies of a Hf containing steel have produced improvements in both creep and corrosion properties of 9%Cr steels. Hf has been ion implanted into thin foils of a 9 wt-%Cr ferritic steel to study its effect on precipitation. Two new types of precipitates are formed, Hf carbide, (an MX type precipitate) and a Cr–V rich nitride, with the formula M₂N. The Hf carbide particles were identified using convergent beam diffraction techniques, and micro-analysis. The nanosized particles are present in much higher volume fractions when compared to VN volume fractions in conventional power plant ferritic steels. Furthermore it is confirmed that the Hf causes the removal of M₂₃C₆ grain boundary precipitates. This has led to an increased concentration of Cr within the matrix, reduced chromium depleted zones at grain boundaries, and increased resistance to intergranular corrosion cracking.     

Phase evolution of AISI 321 stainless steel during directional solidification

Abstract

The phase evolution of AISI 321 stainless steel was studied by directional solidification and quenching techniques. Two interfaces, solid/liquid and the peritectic reaction interface, were found to exist in the directional solidification structure. With increasing growth velocity the solid/liquid interface changed in the sequence of planar, cellular, dendritic and the primary phase changed from austenite to ferrite. The phase and morphology selection was verified by the interface response functions (IRFs) and the maximum growth temperature criterion. The ferritic island banding structure was observed, not only in the austenite cellular primary growth condition (3 μm s^?1), but also in the dendritic ferrite primary growth one at relatively low growth velocity (5 μm s^?1). It is deemed that the former resulted from the nucleation of ferrite in the continuous matrix of austenite phase, yet the latter is the residual primary ferrite attributed to the growth of austenite. Both of them do not come from the nucleation near the solid/liquid interface.     

Structure property correlations in hot isostatically pressed AISI-304 stainless steel

Abstract

Stainless steel components are usually fabricated by conventional manufacturing methods. This route becomes uneconomical for highly intricate shapes. Therefore, processing of AISI-304 stainless steel has been carried out by powder metallurgy and hot isostatic pressing (HIP) route. The microstructure and mechanical properties of the hipped steel produced from inert gas atomised powder were studied. The steel has shown a homogeneous and fine grained microstructure containing annealing twins and the absence of undesirable prior particle boundaries. This has led to higher yield strength, ultimate tensile strength and the ductility than that of the conventionally processed steel. The tensile deformation behaviour of hipped steel was found to be similar to that of the wrought material. Impact strength and fracture toughness of the hipped steel were superior to that of the wrought material; hence the hipped steel could be recommended for manufacture of components for critical applications.     

烧结混合料制粒过程的数值模拟

通过离散元(DEM)法数值模拟软件对烧结混合料在圆筒中的制粒过程进行了数值模拟。模拟结果表明,圆筒混合机的适宜制粒参数为:充填率10%左右,圆筒转速满足N/N临=0.3左右。圆筒转速和充填率过大或过小均对制粒不利,适宜制粒参数的选择应该以物料产生层状瀑布式运动为标准进行判断。另外,模拟结果还表明,制粒主要包括两个过程:一个是颗粒长大,一个是颗粒破坏。当前者占主导时,制粒才能进行,当后者占主导时,制粒效果变差。颗粒的长大主要依靠颗粒滚动,而颗粒的破坏主要是由颗粒之间以及颗粒与筒壁之间的碰撞所致。     

Enhanced sintering and mechanical properties of 316L stainless steel with silicon additions as sintering aid

Abstract

Compaction, effect of ball milling, vaccum sintering, microstructures, volume shrinkage, interconnected porosity, thermal reactions and mechanical properties of 316L stainless steel with and without additions of elemental silicon have been investigated. It was found that the silicon addition enhanced the sintering process by providing a series of liquid phase reactions with the base powder which took place at temperatures below their melting points and the normal solidus range for stainless steels. Differential thermal analysis confirmed formation of liquid phases at three different temperatures which are believed to be responsible for the enhanced sintering process.The first two appeared at ~1060 and 1155°C by two exothermic peaks and the third one at ~1190°C by an endothermic peak. The ball milling operation provided higher green and sintered densities resulting in better mechanical properties due to less agglomorations with finer and much more uniform particle size distribution. Sintered densities of up to 7·44 g cm^-3 with tensile strength of 482 MPa, hardness value of 153 HV10 and 15% elongation were obtained with ball milled plus 3 wt-%Si addition. Low levels of interconnected porosities (~4%) were recorded within the temperature range 1250-1300°C suggesting the possibility of good corrosion resistance.

The sintered microstructures consisted of ferrite and austenite (duplex structure), complex silicide and eutectic phases within grains and at grain boundaries, pools of liquid (rich in Si) and some medium and small pores preventing full density to be achieved despite the liquid phase formation.     

MIM支持块的充模流动计算机模拟

以流体力学和热力学理论为基础，建立了充模流动过程的控制方程、初始条件和边界条件，并以支持块的充模流动为例，采用大型通用有限元软件ANSYS进行了计算机模拟，着重研究了浇口位置和浇口尺寸大小对压力场，温度场的影响，同时结合实验分析了转角处缺陷产生的机理。模拟结果表明：当采用尺寸大小为4mm的中间浇口进行注射时，可以得到良好的支持块预成形坯，与实验结果一致。