Tom Bell Young Author Award Winner 2008 Influence of silicon on nitriding behaviour of hot work tool steels

Abstract

Hot work tool steels are widely used for pressure die casting moulds, die inserts, extrusion tools for aluminium processing and for steel forging. Nitriding increases the lifetime of such tools in many cases, yet delivers disappointing results in others. To optimise performance and for knowledge based surface design, it is indispensable to understand the mechanisms which occur in the near surface zone during nitriding. Nitrogen and carbon profiles obtained for X38CrMoV51 (AISI H13) steels with two silicon levels (1·1 and 0·3%), together with high resolution microscopy and electron energy loss spectroscopy, revealed that the secondary carbides are gradually transformed into nitrides during nitriding. Thermodynamic calculations confirmed the experimental observations. The near surface zone can be divided into three subzones: (1) a nitrogen enriched, almost carbon free zone with high nitride precipitation density and high hardness; (2) a nitrogen enriched and carbon depleted zone where the carbide–nitride transformation occurs; (3) a carbon enriched zone where the displaced carbon from zones 1 and 2 reprecipitates. A correlation between microstructure and microhardness and residual stress profiles was observed for all three zones. It was found that silicon, although not directly participating in the formation of nitrides, has a strong impact on the properties of the near surface zone by stabilising the secondary carbides and retarding the carbide–nitride transformation. This results in homogeneous precipitation in the transformation zone, thus avoiding micrometre sized precipitates which can act as defects and promote crack propagation. The conclusions of the present work are in accordance with literature studies on the effect of silicon on the tempering behaviour and the secondary carbide structure of 5%Cr martensitic steels.     

Active screen plasma nitriding of materials

Abstract

The rapid development and the uptake of plasma nitriding technology into industrial surface engineering have slowed down in recent years. This is attributed to some of the inherent shortcomings of conventional dc plasma technology, for example, difficulties in maintaining a uniform chamber temperature, instability of the plasma and potential surface damage to parts caused by arcing. Efforts in overcoming these problems have led to the development of active screen plasma nitriding (ASPN) technology. This review demonstrates that with all its technological and environmental advantages, ASPN can be used to treat low alloy steels, tool steels, stainless steels and other steels which can conventionally be nitrided with dc plasma technology. In addition, ASPN can be used to treat non-conducting materials such as oxidised steels and polymeric materials which are not suitable for a dc plasma nitriding system. In the longer term, environmental friendly and technologically advanced plasma nitriding will outperform conventional salt bath and gaseous methods. Active screen plasma nitriding is a novel way of possibly achieving the full potential of plasma technology for thermochemical surface engineering.     

Effect of lanthanum rare earth addition on low temperature plasma nitriding

Abstract

The effect of lanthanum additions on the plasma nitriding of nanocrystalline steel 3J33 is reported. Nitriding was carried out in 20N₂ + 80H₂ at a pressure of 400 Pa at 350 and 410°C. Glow discharge spectrometry shows that although its atomic radius is larger than that of Fe, La can diffuse into the surface layer to a significant depth; the La addition increases the compound layer thickness. Based on the microhardness profile, the diffusion layer thickness also increases due to La addition. XRD analyses indicate that the presence of La did not change the phase structure of the surface, i.e. a compound layer of γ′-Fe₄N type. The incorporation of La not only enhances the microhardness but also improves the toughness of the nitrided layer. The atomic and electron structures of phases γ′-Fe₄N and γ′-(Fe,La)₄N were calculated using the plane wave pseudopotential method. The (Fe,La)₄N phase is more stable than Fe₄N and the shear/bulk modulus ratio G/K of (Fe,RE)₄N is much smaller. This finding provides a possible mechanism for the increased microhardness and improved toughness of nitrided layers containing La.     

Influence of oxygen in plasma nitriding

Abstract

The development of low temperature nitriding treatments to enhance wear resistance over the past 40 years is reviewed. The progress achieved is illustrated with evidence from extensive laboratory investigations and industrial trials on textile machine components and other parts. It is demonstrated that small additions of an oxidising component to gaseous nitrocarburising or plasma nitriding/nitrocarburising atmospheres have a beneficial effect on layer formation and on the wear properties of the treated components. The presence of oxygen increases layer formation rates and stabilises the ?-compound layer. The role of oxygen on surface activation is important for chromium containing steels but also on plain carbon or alloyed steels and to neutralise the passivating effect of finishing procedures. Applying a post-oxidation step after nitrocarburising can significantly increase the corrosion resistance of the surface but close control of gas composition and treatment parameters is necessary.     

Internal-Nitriding Behavior of Ni-V and Ni-3Nb Alloys

Ni-2V, Ni-5V, Ni-12V, and Ni-3Nb alloys (w/o)were nitrided in 10 v/o NH₃(bal.H₂) over the range of 700-1000°C. Thegrowth rates of the reaction zones followed parabolicbehavior for all of the alloys from 700 to 900°C. At 1000°C, Ni-2V andNi-3Nb formed nitride scales, whereas Ni-5V and Ni12Vformed internal-nitride zones. Nitridation ratesdecreased with increasing vanadium content for the Ni-Valloys. VN precipitated in the Ni-V alloys and NbNprecipitated in Ni-3Nb for all exposure conditions inwhich internal nitridation occurred. The precipitatemorphology changed with temperature and distance from the gas-metal surface. The VN and NbNprecipitates were generally small and spheroidal nearthe surface, increasing in size with distance andtemperature. The NbN precipitates became Widmanstattenat higher temperatures and/or increasing distance withinthe nitrided zone. The solubility of nitrogen in pure Niwas determined and found to decrease with increasingtemperature from 700 to 1000°C. Expressions for the diffusion coefficient of nitrogen in nickelwere determined from the measured permeabilities of eachalloy and the nitrogen solubilities in nickel.     

Nitrogen diffusion and nitrogen depth profiles in expanded austenite: experimental assessment,numerical simulation and role of stress

Abstract

The present paper addresses the experimental assessment of the concentration dependent nitrogen diffusion coefficient in stress free expanded austenite foils from thermogravimetry, the numerical simulation of nitrogen concentration depth profiles on growth of expanded austenite into stainless steel during gaseous nitriding, a qualitative discussion of the role of stress on local equilibrium conditions of growing expanded austenite and a discussion of the erroneous concentration dependent diffusivity of nitrogen in expanded austenite as obtained from applying the Boltzmann–Matano method to composition–depth profiles.     

Fabrication of 6061 aluminium alloy/Al18B4O33(w) composite by using sol–gel alumina binder

Abstract

For fabrication of aluminium borate whisker (Al₁₈B₄O_33(w)) reinforced 6061 aluminium alloy composites, a sol–gel alumina binder instead of conventional silica binder was used for preparing the whisker preforms of the squeeze cast composites. The results show that a sound whisker preform and a uniform composite can be made by this method. Unlike the reactive silica binder, the sol–gel alumina binder is rather stable throughout the entire high temperature fabrication process. Under appropriate conditions, the sol–gel alumina binder can also serve as a thermal barrier for minimising interfacial reactions between aluminium borate whiskers and the matrix alloy. With a binder concentration of 0.6 mol L^-1, the ultimate tensile strength of the composite is as high as 277.6 MPa at room temperature and moderate at elevated temperatures. The tensile fracture of the alumina bound composite shows a mixed mode of dimple fracture and interface debonding.     

Effects of addition of TiN nanoparticles on microstructure and mechanical properties of TiC based cermets

Abstract

Titanium carbide based cermets with additions of titanium nitride nanoparticles were fabricated by using ultrasonic dispersion and conventional powder metallurgy techniques. In order to characterise the microstructure, SEM and TEM have been utilised. The results show that the microstructure becomes finer with increasing additions of titanium nitride nanoparticles. The mechanical properties were also measured and the results show that the properties do not change monotonously with the amount of titanium nitride nanoparticle addition. Good properties can be obtained when the amount of TiN addition reaches 6 - 8 wt-%. The improvement of properties is mainly owing to the refinement of TiC grains as well as the deflection effect of titanium nitride nanoparticles on crack propagation.     

Effect of calcium on primary silicon particle size in hypereutectic Al–Si alloys

Abstract

Modification of hypereutectic Al-Sialloy, B390 alloy for the refinement of primary silicon particles, and its effects on tensile and impact properties were examined. Calcium was found to have an effect on the size of primary silicon particles. Primary silicon particle size was refined as calcium content decreased. Control of calcium content by the addition of Ti₂Cl₆ to the melt resulted in successful refinement of primary silicon particles. The minimum size of primary silicon particles was 20.3 μmwitha residual calcium content of 16 ppm. The microstructure was composed of very fine 20.3 μm primary silicon particles, compared to 24.5 μm primary silicon particles obtained using the AlCuP method, previously reported as the most effective method. Refinement of primary silicon particles led to an improvement in the mechanical properties of the alloy, especially elongation.     

The Internal-Nitriding Behavior of Co-Fe-Al Alloys

Co-10Fe, Co-20Fe, and Co-40Fe alloys containing3 at.% Al were internally nitrided inNH₃/H₂ mixtures over the range700-1000°C. The kinetics of thickening of theinternal-reaction zone followed the parabolic rate law, suggesting thatsolid-state diffusion was rate controlling. Nitrogenpermeabilities were obtained for each alloy. AlN was theonly nitride to form for all materials and at alltemperatures. At high temperature, the nitride precipitatesformed hexagonal plates near the surface, theprecipitates becoming more blocky near the reactionfront. Precipitate size increased with increasing depthin the alloy and increasing temperature, becauseof competition between nucleation and growth processes.Increasing iron content increased the reacion kineticsdue to increased nitrogen solubility with increasing iron content.