Influence of carburising on distortion behaviour

Case hardening is a common process to produce steel components, which are characterised by a hard surface combined with a ductile core. As a result of this treatment the parts show a good wear resistance as well as an increase in fatigue strength. High strengthened gear wheels can be mentioned as important examples, being treated in this manner. The case hardening process consists of a carburising and a hardening treatment. The carbon gradient in the surface results in a gradient in the transformation behaviour. The transformation to ferrite, pearlite and bainite is affected as well as the martensite transformation. Distortion due to these chemical inhomogeneities is added to distortion caused by temperature gradients and temperature gradient implied transformation stresses. In this project the effect of the carburising process on distortion behaviour is investigated. The influence of the microstructure before carburising, the carburising depth, the surface carbon content, the course of the process, the carburising temperature, the hardening temperature and the interaction between these parameters are investigated. Design of experiment methods are used to receive the effect of interactions between the varied parameters and to reduce the number of experiments. Additional investigations concern parameters, which can not be varied completely with the other parameters. The influence of low pressure carburising treatments and carburising at high temperatures, for example, are analysed in a spot‐check. The aim of the investigations is to find the decisive parameters affecting distortion in the carburising process, which are later on varied in a design of experiments plan containing the whole production line.     

Mikrostrukturelle Erscheinungen in der Randschicht einsatzgehärteter Bauteile

Microstructural peculiarities in the surface of case-hardened components The negative influence of the surface-near structure of case-hardened components as caused by the pick-up of non-metallic elements from the carburising atmosphere is a well-known fact. The most prominent effect produced by oxygen is the so-called internal oxidation, which reduced by oxide formation the amount of dissolved alloying elements manganese, chromium and silicone, resulting in a respective hardenability loss. The present work presents microscopical and spectroscopical possibilities for specifying the different forms of appearance of the internal oxidation, as well as critical reflections on their evaluation. In addition, a somewhat less known, negative influence of the loss of hardenability inflicting alloying elements on the outer part of the case structure is mentioned, which is found in low-pressure carburising with or without plasma support.     

Über den Zusammenhang zwischen dem Kohlenstoffgehalt in Stählen und der Härte des Martensits

About the interrelation between carbon content in steels and the martensite hardness The hardenability of steels, identified by the interrelation between carbon content and available maximum hardness in a full martensitic microstructure is important for decision in case of steel selection and optimal construction of components. In the industrial practice formulas for calculation were used, deduced from the classic statements of Hodge/Orehoski and Burns/Moore/Archer. But the calculated results are significantly different. For a more exactly view datas of the “Atlas zur Wärmebehandlung der Stähle” were used. The better results are to expect by using the formula of Just. The application is also important for case hardening. It can be concluded that a lower carbon content than 0,35 mass‐%, the value usual is used, is sufficient to arrive a limit hardness of 52.5 HRC or 550 HV for determination of the case hardening depth. This confirms with the investigations of the AWT‐Fachausschuß 5/AK4. There has been found a formula of interrelation between carbon content and martensite hardness that gives a curve identical with the curve from Burns/Moore/Archer.     

A study of self‐hardening bainitic cast steel

Bainitic cast steel is a kind of wear resistant material which has high strength and toughness, and can usually be obtained by isothermal quenching or molybdenum alloying. However, isothermal quenching has lower production efficiency and molybdenum alloying has higher production cost. In this paper, according to the characteristics that manganese and boron elements delayed the pearlitic transformation, the authors developed a new type of self‐hardening bainitic cast steel in which manganese and boron were main alloy elements and a small amount of titanium, nitrogen, calcium, barium and yttrium elements were also added in the steel that could refine and purify the solidification structure of steel. On this basis, the author studied the effect of tempering treatment on microstructures, mechanical properties and wear resistance of bainitic cast steel. The results showed that impact toughness of bainitic cast steel increased ceaselessly with the increase of tempering temperature, and there was tempering brittleness while tempering from 450°C–500°C. Moreover, the hardness of bainitic cast steel decreased with the increase of tempering temperature, and hardness decreased slowly and maintained at 55HRC or above when tempering temperature was lower than 300°C. Under the condition of two‐body pin‐on‐disc wear, the wear resistance of bainitic cast steel decreased with the increase of tempering temperature, but bainitic cast steel tempering at 300°C had excellent wear resistance in the condition of impact wear. In the practical use, the bucket teeth of excavator and the hammer of crusher making from self‐hardening bainitic cast steel were safe and reliable, and their service life were increased by 120–150% than Hadfield manganese steel.     

Metallurgical influence on distortion of the case‐hardening steel 20MnCr5

In literature, only few works are published that make statements on influences between material properties and distortion. There are some investigations on the influence of carbides and their distribution in the structure. Other authors report on a distinct influence of the texture on distortion. However, the operational sequences during solidification as well as the following deforming processes affect the homogeneity of the chemical composition. Local fluctuations of the chemical composition lead to locally different phase transformations. As a result there will be differences in the structure and therefore inhomogeneous distortion. The hardenability could be suitable as an integrating parameter to describe the effect of the alloying elements as well as their homogeneity in distribution in the structure on distortion. Additionally, the preheating treatment of the material as well as the carburizing process can influence the size alteration and the deformation. Therefore, in this work steel of the quality 20MnCr5 with different hardenability grades were varied concerning their preheating treatment conditions. Three conditions were adjusted: annealing to a specific structure, pre‐aging and the condition after cooling from the hot rolling heat. The extend of distortion was evaluated by measurements on a coordinate measuring apparatus before and after hardening.     

An approach for modeling the active transformation of microstructure of two‐phase Alloys in FEM simulation of technological chains in superplastic forming (SPF)

Optimization and broadening of the technological regimes of superplastic forming require more accurate accounting of the transformation of microstructure during all stages of the forming process. On the one hand, it is very important to predict the resultant microstructure in different parts of the formed structure as it determines the operation properties, and on the other hand, active transformation of microstructure can lead to either hardening (grain coarsening) or softening (grain refinement) of the material. These changes in the mechanical behavior of a material can be quite significant and need to be taken into account in the FEM simulation of superplastic forming processes to achieve the required accuracy. One approach for modeling the microstructural transformation in an FEM simulation for a two‐phase alloy is proposed herein. Constitutive model with internal variables is described. The questions of validity of macroscopic and microscopic equations are discussed. The approach for taking into account the transformation of metallographic texture is proposed.     

Physicomechanical properties of hardened copper-aluminum-zincβ-alloys with an intermediate structure

The hardenability for two hardening rates (water and air) and also the influence of diffusion transformation products on their damping capacity and mechanical properties are investigated in a broad concentration area of copper-aluminum-zinc system alloys. The general character of the change in microstructure and hardness with distance from the specimen surface is determined. The relationship of hardenability of alloys of the investigated system to their composition for two hardening rates is established. It is shown that with water hardening these alloys possess high and with air hardening insufficient hardenability. It is determined that formation in hardening in the structure of the alloys of products of diffusion decomposition of the matrix phase reduces their damping capacity and mechanical properties.Translated from Problemy Prochnosti, No. 11, pp. 66–70, November, 1992.     

TRIP and phase evolution for the pearlitic transformation of the steel 100Cr6 under. step‐wise loads

The investigation of complex material behaviour of steel like transformation‐induced plasticity (TRIP) and stress‐dependent phase transformation (SDPT) is a large field of current research. The simulation of the material behaviour of work‐pieces in complex situations requires a knowledge as deep as possible about such phenomena. In addition, there are effects in the case of non‐constant stress which cannot be explained by the widely used Leblond model for TRIP. Therefore, we consider a TRIP model taking into account back stress due to TRIP itself. Based on experimental data for the isothermal pearlitic transformation of the steel 100Cr6 (SAE52100) under step‐wise loads we calculate material parameters for the extended TRIP model. Regardless of the preliminary character of the performed tests, all experiments show a back‐stress effect with a decrease of the TRIP strain after unloading.     

4．5Ni钢淬透性试验研究

通过４．５Ｎｉ钢淬透性曲线、等温转变曲线及常规力学性能试验，研究了４．５Ｎｉ钢的淬透性；同时分析了合金元素对淬透性的影响；并用临界冷却速度预测了厚板能否淬透。研究结果表明，由于合金元素的交互作用，４．５Ｎｉ钢的淬透性优良，不但８０ｍｍ厚板可以淬透，而且只要严格控制化学成分，同时提高淬火剂的冷却强度，厚度不大于１００ｍｍ的板材也能淬透。     

The variation of strain rate sensitivity exponent and strain hardening exponent in isothermal compression of Ti–6Al–4V alloy

The deformation behavior in isothermal compression of Ti–6Al–4V alloy is investigated in the deformation temperatures ranging from 1093 K to 1303 K, the strain rates ranging from 0.001 s⁻¹ to 10.0 s⁻¹ at an interval of an order magnitude and the height reductions ranging from 20% to 60% at an interval of 10%. Based on the experimental results in isothermal compression of Ti–6Al–4V alloy, the effect of processing parameters and grain size of primary α phase on the strain rate sensitivity exponent m and the strain hardening exponent n is in depth analyzed. The strain rate sensitivity exponent m at a strain of 0.7 and strain rate of 0.001 s⁻¹ firstly tends to increase with the increasing of deformation temperature, and maximum m value is obtained at deformation temperature close to the beta-transus temperature, while at higher deformation temperature it drops to the smaller values. Moreover, the strain rate sensitivity exponent m decreases with the increasing of strain rate at the deformation temperatures below 1253 K, but the m values become maximal at a strain rate of 0.01 s⁻¹ and the deformation temperature above 1253 K. The strain rate affects the variation of strain rate sensitivity exponent with strain. Those phenomena can be explained reasonably based on the microstructural evolution. On the other hand, the strain hardening exponent n depends strongly on the strain rate at the strains of 0.5 and 0.7. The strain affects significantly the strain hardening exponent n due to the variation of grain size of primary α phase with strain, and the competition between thermal softening and work hardening.     

Klebflächenvorbehandlung von Aluminiumoberflächen mit Haftvermittler

Surface pretreatment for adhesive bonding of aluminum with adhesive mediator The influence of three steps pretreatment degreasing, blasting and primer and the two steps pretreatment degreasing and blasting on the adhesive behaviour of aluminum alloy AlMg4,5Mn0,5 were investigated. The investigations were preformed using two cold hardening two‐components epoxy adhesives, one hot hardening one‐component epoxy adhesive and one cold hardening two‐components polyurethane adhesive. The three steps pretreatment indicate that the adhesion bonding strength by epoxy adhesives are higher than that by two steps pretreatment. On the other hand, the adhesion bonding strength was similar by using polyurethane adhesive independence of the pretreatment method. Leaving out the pretreatment step degreasing after corund blasting caused by using, PU, EP 2 and EP 3 adhesives a significant decreasing of the bonding strength in comparison with the only degreased specimen. This degreasing process was used to remove the residual blasting medium from the aluminum surfaces after blasting. The topographical structure of the surfaces after corund blasting was covered by further pretreatment with primer as a consequence of higher primer viscosity, which causes a decreasing in the surface roughness. The chemical composition of pretreated surfaces by three steps was different from that by two steps.     

Abrasive wear property of bainitic nodular cast iron in laser processing

The effects of the different austenitizing and isothermal transformation temperature and time on the wear resistance in bainitic ductile iron has been investigated and compared with surface hardening by laser processing, to find the best wear resistance for the bainitic ductile iron. It was found that the wear resistance of the lower bainitic ductile iron was better than upper bainitic ductile iron, and that the factor affecting hardness and wear properties the most was the isothermal transformation temperature. After surface hardening by laser processing, the hardening reached RC 55 or more, no evidence of any difference between the lower and the upper bainitic ductile iron being found.     

Untersuchungen zur Wälzermüdung nach induktivem Kurzzeitanlassen

Ball bearing fatigue after inductive short time tempering To improve toughness, machiniability and to avoid distortion under operating conditions inductive hardened components often were additional tempered in an oven. As a general rule tempering is applied when grinding is required. According to tempering conditions the present hardness, toughness and the microstructure will change. In the present studies it is shown, that with optimized inductive short term tempering conditions it was possible to achieve even better mechanical properties in comparison with oven tempered components. Ball bearing fatigue testings showed an improvement of wear resistance. The achieved findings opens the possibility for numerous applications to exchange the previous oven tempering through the more rapid and more low‐cost inductive short‐term inductive surface layer hardening.     

TENSILE AND LCF PROPERTIES OF AISI 316LN SS AT 300 AND 77 K

Tensile and low-cycle fatigue tests were performed on a 316LN austenitic stainless steel at 300 and 77  K. The tensile and low-cycle fatigue properties were obtained and analysed in terms of the influence of temperature on the plastic deformation process and the formation of strain-induced martensite. The martensite content was evaluated using measurements of magnetic saturation. No α'-martensite was detected at 300  K under either monotonic or cyclic straining. On the contrary, at 77  K, strain-induced martensitic transformation is responsible for the higher elongation in tension and the secondary hardening observed on hardening/softening curves in low-cycle fatigue. The induced martensite content in tensile tests is a function of strain which deviates from Angel's model. In low-cycle fatigue, it is a function of the strain level and the accumulated plastic strain. At a given total strain amplitude, the decrease of temperature from 300 to 77  K results in the decrease of plastic strain amplitude and homogenization of plastic strain distribution, and thus in the prolongation of fatigue life. The cyclic over-stress at 77  K, due to an intermediate ageing at 300  K, is related to pinning of initially free dislocations resulting from nitrogen diffusion during isothermal holding at room temperature. This results in a reduced fatigue life.     

Klebflächenvorbehandlung von Aluminiumoberflächen mit SIP‐Haftvermittler

Surface pretreatment for adhesive bonding of aluminium with adhesive mediator SIP The present contribution describes the influence of different surface pretreatments including adhesive mediator SIP for adhesive bonding of aluminium alloy AlMg4,5Mn0,4. The investigations were performed using two cold hardening two‐components epoxy‐adhesives, one hot hardening one‐component epoxy‐adhesive and one cold hardening two‐components polyurethane‐adhesive. The adhesive bonds with epoxy‐adhesives show after three‐step pretreatment degreasing + corundblasting + SIP coating the highest adhesive strength values whereas adhesive bonds with polyurethane‐adhesive showed a decrease of bond strength as compared with the delivering surface condition.     

Vakuumwärmebehandlung im Zweikammerofen

The aim of the latest research and development works on the field of vacuum heat treatment is the realisation of higher quenching intensities. The increase of quenching speeds makes it possible to extend the range of workpieces which can be heat treated in vacuum to low-alloyed steels as for example ball bearing-steels, case hardening steels and steels for hardening and tempering and to increase the through hardening to bigger cross sections. On this field the most modern vacuum technology offers an alternative to other heat treatment processes like for example hardening in salt bath.     

Einfluss von Temperatur und Vorverformung auf das plastische Werkstoffverhalten von modernen Karosseriestählen

Influence of temperature and prestraining on the plastic material behaviour of modern sheet steels for autobody applications Within the scope of a common research project of the automotive and steel industry, characteristic values describing the plastical behaviour of 20 sheet steels have been determined. In detail, quasistatic tensile tests at the testing temperatures ‐40 °C, 23 °C and 100 °C were carried out to obtain flow curves for the as delivered materials as well as for steels after a defined prestraining or heat treatment. Additionally, sheet metal testing led to forming limit diagrams and limiting drawing ratios including the working ranges for deep drawing. The results of the tensile tests showed significant differences between steel groups with regard to their strain hardening behaviour, which can be described by the ratio of yield and tensile strength R_p0,2/R_m or the Θ_IV‐value, and their temperature sensitivity. Within one steel group, consisting of steels with similar strain hardening behaviour, it might be possible to determine flow curves of one steel in a defined condition in order to calculate the flow curves of other steels with different strength. An advantage would be a lesser number of experimental tests which have to be performed in order to supply reliable input data for numerical material and component modelling.