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.     

Cracking mechanisms in large size ingots of high nickel content low alloyed steel

This study is focused on determining the possible root causes for cracking after open die forging of large size ingots made of high nickel medium carbon low alloy steels. Optical and scanning electron microscopies as well as Energy Dispersion Spectroscopy (EDS) were used to examine the microstructure of the samples taken out of a cracked forged ingot. The large size of the samples permitted to investigate microstructure at different locations at the surface and in depth. Chemical analysis revealed chromium and oxygen enrichment at the grain boundaries. Grain size measurement indicated clear differences between “clean” surface zones and cracked ones, and between surface and in depth regions. The analyses indicated that fracture phenomena may be due to abnormal grain size which promotes oxide penetration into grain boundaries, resulting in their embrittlement and cracking upon cooling.     

Regression-based CVN–KIC Models for hot work tool steels

Dies and tools used in hot metal forming (extrusion, forging, rolling, etc.) are exposed to high pressures, elevated temperatures, and thermo-mechanical fatigue. The most common mode of in-service die failure is fatigue fracture (brittle failure through crack propagation). Reliable determination of fracture toughness of the die material is thus critically important. However, as die steels have a combination of high-hardness and high-strength, and are used at elevated temperatures, standard plane-strain fracture toughness (K_IC) testing methods become impracticable. Alternate testing procedures such as the Charpy impact energy (CVN), together with empirical/semi-empirical correlations of K_IC to other data, are then more viable and economical. Experimental data (values of K_IC, CVN, and HRC) of H13 steels have been collected through an exhaustive literature search. This data set has been augmented through in-house experimentation: samples variously heat treated (different tempering temperatures and times, and both air-cooling and oil-quenching), and tested at different working temperatures. Linear and quadratic models are proposed for determination of fracture toughness, based on experimental (in-house) and published values of Charpy impact energy (CVN) and Rockwell hardness (HRC), both at room and at elevated temperatures.     

Werkstoffverhalten und Mikrostrukturentwicklung hochfester Mn‐Al‐Si‐Leichtbaustähle unter Zugbelastung

Material behaviour and microstructure evolution of high‐strength Mn‐Al‐Si‐light‐weight steels under tensile loading Because of their extraordinary combination of high strength and superior ductility high‐strength high Mn‐steels with reduced density and additions of aluminium and silicon are interesting candidates for structural applications. The initial microstructure consisting of stable austenite or austenite and ε‐ and α'‐martensite was achieved by alloying. During plastic deformation intense strain induced martensitic transformation and / or mechanical twinning was observed. These deformation mechanisms are used to extend the limited forming capability and contribute to a high energy absorption (in impact tests) up to very high strain rates. Tensile tests reveal that the properties are maintained up to strain rates of about 1000 1/s. The flow stress behaviour is strongly influenced by the initial microstructure and their evolution during deformation processes is determined by the rates of martensite and twin formation, respectively.     

Experimental and analytical studies on the prediction of forming limit diagrams

Metal forming processes are widely used in industrial productions, automobile bodies, food industries, oil refineries, and liquid and gas transmission systems. Analyzing these processes is very important to reduce wastes and optimize the processes. Study of some main factors such as physical and mechanical properties of material and its formability, die geometry, die material, lubrication and pressing speed has been the topic of many research projects. In this paper, forming limit diagrams (FLDs) for LC and ULC steels and the effect of different parameters like the work-hardening exponent, n, and the plastic strain ratio, r, on these diagrams have been evaluated and simulated using ABAQUS/Standard. In this case, Hill’s quadratic anisotropy function is assumed to be the yield function and the Atkins criterion is used as the failure criterion.     

New Optimum Extrusion Parameters of 9% Cr Ferritic Steel Tubes

The mechanism of austenite grain growth prior to forging of 9% Cr ferritic steels was analyzed. Pre-forging heating temperature and holding time had an obvious effect on austenite grain size and delta ferrite morphology. Hot compression experiments were conducted by using a Gleeble 1500D thermal–mechanical simulator. A hot upsetting test was performed on an HP01-500 oil press to verify the geometric dynamic recrystallization mechanism. New extrusion technology parameters of steel tubes were optimized by the Deform 2D software, and practical tests were conducted. Results indicated that the grains grew slowly at low temperatures (i.e., 1000°C and 1050°C). The kinetic curve of grain growth was close to the “S” type at 1100°C. The grains exhibited abnormal grain growth at high temperatures (i.e., 1150°C and 1200°C). The quantity of delta ferrites markedly increased at 1250°C. In addition, grains were refined by geometric dynamic recrystallization during the formation process. Mechanical properties of 9% Cr tube met standard requirements.     

Microfractography in failure analysis of cold forging dies

In this paper microfractographic features in fracture surfaces for tensile, fatigue, impact and three point bending of cold forging die steels with Rockwell C scale hardness number of 52–68 is presented. The emphasis is placed upon the stretched zone formation ahead of fatigue crack and the relation between the stretched zone width and fracture toughness of these cold forging die steels. Finally it is briefly described that the quantitative analysis for cold forging die failure can be possible by measuring the stretched zone width.     

Lifetime‐determining spalling mechanisms of NiCoCrAlRE / EB‐PVD zirconia TBC systems

The mechanisms that control the lifetime of thermal barrier coating (TBC) systems have been traced by two particular overlay bondcoats serving as model systems: superalloy pins (IN100, CMSX‐4) with two alternative NiCoCrAlRE (RE: Hf, Y) bond coat compositions (i) NiCoCrAlY without and (ii) with co‐dopants of silicon and hafnium. On top an electron‐beam physical‐vapor deposited (EB‐PVD) yttria partially stabilized zirconia (YPSZ) TBC commonly mixed with 2 wt.% hafnia, or, rarely with 10 wt.%, was applied. The test pins were thermo‐cycled at 1100 and 1150 °C until failure. Identical lifetimes in cyclic tests on YPSZ TBCs with 2 (relatively high sintering rate) and 10 wt.% hafnia (relatively low sintering rate) preclude an effect of diffusion mechanisms of the YPSZ TBC on lifetime. The fit of lifetimes and test temperatures to Arrhenius‐type relationships gives activation energies for failure. These energies agree with the activation energies for anion and cation diffusion in alumina for the respective bondcoat variant: (i) for the NiCoCrAlY/TBC system for O^2‐ diffusion in alumina, (ii) for the NiCoCrAlYSiHf/TBC system for Al³⁺ diffusion in alumina. SEM and EDS investigations of the thermally grown oxides (TGOs) confirm the mechanisms responsible for TBC failure as indicated by activation energies. Two categories of failure can be distinguished: (i) NiCoCrAlY coatings fail by an “adhesive mode of failure” along smooth bond coat/TGO interfaces driven by a critical TGO thickness. (ii) NiCoCrAlYSiHf coatings fail later and more reluctantly by a “cohesive” crack mode via de‐cohesion at the TGO/TBC interface. In the latter case a quasi‐integrity of the crack‐affected TGO is lengthily maintained up to failure by a crack‐pinning mechanism which runs via Al³⁺ supply from the bondcoat.     

Quantification and microstructural origin of the anisotropic nature of the sensitivity to brittle cleavage fracture propagation for hot-rolled pipeline steels

This work proposes a quantitative relationship between the resistance of hot-rolled steels to brittle cleavage fracture and typical microstructural features, such as microtexture. More specifically, two hot-rolled ferritic pipeline steels were studied using impact toughness and specific quasistatic tensile tests. In drop weight tear tests, both steels exhibited brittle out-of-plane fracture by delamination and by so-called “abnormal” slant fracture, here denoted as “brittle tilted fracture” (BTF). Their sensitivity to cleavage cracking was thoroughly determined in the fully brittle temperature range using round notched bars, according to the local approach to fracture, taking anisotropic plastic flow into account. Despite limited anisotropy in global texture and grain morphology, a strong anisotropy in critical cleavage fracture stress was evidenced for the two steels, and related through a Griffith-inspired approach to the size distribution of clusters of unfavorably oriented ferrite grains (so-called “potential cleavage facets”). It was quantitatively demonstrated that the occurrence of BTF, as well as the sensitivity to delamination by cleavage fracture, is primarily related to an intrinsically high sensitivity of the corresponding planes to cleavage crack propagation across potential cleavage facets.     

Acoustic Emission (AE) Monitoring of Open Die and Closed Die Forging Processes of Al Alloy

Acoustic emission (AE) generated during open die and closed die forging processes of Al alloy has been studied. AE generated during both types of the forging processes could be differentiated into three stages, namely, yielding of the workpiece material upon start of forging, intermediate deformation and filling of the die. The effect of lubrication on the AE generated during open die forging was also studied. This showed that AE increases with decreasing lubrication of the workpiece thus enabling AE to detect lubrication inadequacy/breakdown during the forging process.     

三销轴叉精整底部裂纹产生和防护

三销轴叉精锻件是汽车等速传动系统中的关键零件,其头部内、外腔形状复杂,尺寸精度要求高,故一般采用温锻预成形+冷精整成形复合成形技术,而预成形温锻件设计不合理,在冷精整成形中,存在内腔底部薄壁处出现裂纹的问题。利用数值模拟技术,分析了三销轴叉冷精整型腔底部区域成形过程的成形力、应力、应变的变化,对预成形温锻件进行了优化,设计了不同的入模角,入模角的起始点位于内腔基准平面之上。工程实践后,优化后的预成形温锻件在冷整形后,口部内腔底部没有裂纹产生,流线连续,获得较为理想的精锻件。证明优化后的预成形温锻件,经冷精整成形后,精锻件产品质量显著提高,可有效防止裂纹产生。     

Investigation on a new process chain of deposition or friction welding and subsequent hot forging

According to the state of the art most current forging parts and technical components are made of mono‐materials. Nevertheless, parts consisting of only one material increasingly reach their specific material and constructive limits in the established production processes. Through use of previously joined raw parts consisting of different materials, it is possible to produce application‐optimized hybrid parts. This paper describes the production chain of hybrid parts produced by combining two different joining processes with subsequent hot compression tests. The joining of various materials is realized by a deposition welding with a laser‐stabilized gas‐metal‐arc deposition welding (LGD) process and a conventional friction welding process. Subsequently, the hybrid samples are compressed under varying forming parameters such as temperature and deformation degrees. In order to characterize the joining zone, metallurgical investigations are carried out.     

Kraft- und Wegmessungen als Grundlage für das Verständnis des Kerbschlagbiegeversuchs

Load-Displacement Diagrams – a Basis for the Understanding of Notched Bar-Impact Tests . The absorbed energy as well as the effect of temperature on absorbed energy during the recording of load – displacement diagrams of notched bar – impact tests of mild steels, Ck 10, Ck 35, Ck 45, Ck 60 and C 85W 2 in normalized state were analysed. The considerable values of relation between discontinuity in nature of characteristic load and displacement of load displacement diagrams and irregularity of fracture toughness temperature curves were obtained there. From these investigations one can define characteristic temperatures (transition temperatures), which are not arbitrary, but physically founded and which describe the behaviour of materials in different temperature-ranges. These characteristic temperatures, above all, the crack arrest temperature could also be defined after fractographical and microfractographical investigations. The absorbed energy temperature curves were also determined arithmetically with the application of simple methode of area computation of load displacement diagrams. Therefore, it can be made sure that the available curves are correct and determined on principle of physics.     

The influence of the nitrogen/nickel‐ratio on the cyclic behavior of austenitic high strength steels with twinning‐induced plasticity and transformation‐induced plasticity effects

Austenitic high nitrogen (AHNS) and austenitic high interstitial steels (AHIS) are of interest for mechanical engineering applications because of their unique combination of mechanical (strength, ductility), chemical (corrosion resistance) and physical (non‐ferromagnetic) properties. But despite their high strength values e. g. after cold deformation up to 2 GPa in combination with an elongation to fracture of 30 %, which is based on twinning‐induced plasticity (TWIP) mechanisms and transformation‐induced plasticity (TRIP) mechanisms, the fatigue limit remains relatively small. While for chromium‐nickel steels the fatigue limit rises with about 0.5‐times the elastic limit it does not at all for austenitic high‐nitrogen steels or only to a much smaller extent for nickel‐free austenitic high‐interstitial steels. The reasons are still not fully understood but this behavior can roughly be related to the tendency for planar or wavy slip. Now the latter is hindered by nitrogen and promoted by nickel. This contribution shows the fatigue behavior of chromium‐manganese‐carbon‐nitrogen (CrMnCn) steels with carbon+nitrogen‐contents up to 1.07 wt.%. Beside the governing influence of these interstitials on fatigue this study displays, how the nitrogen/nickel‐ratio might be another important parameter for the fatigue behavior of such steels.     

Über adiabatic shearbands und die Entstehung der „Steilen Weißen Bänder”︁ in Wälzlagern

About adiabatic shear bands and the generation of “high‐angle white bands” in roller bearings During the rolling motion of roller components structural changes are generated by over‐elastic distressing below the bearing face. Micro‐ and macro‐plastic distortions of the microstructure due to the three‐axial distressing accompany the process of rolling contact fatigue with the start from the first load cycle. They determine the life‐duration of the roller component. Further structural changes in ball bearings besides the plastic deformations are the so called “butterflies” and the so called low‐ and high‐angle “white bands”. The “white bands” you only can detect later, after an extended number of rolling actions. They are inclined to average circumferential angles at ≈ 30 ° respectively ≈ 80 °. Butterflies and white bands develop obviously due to a two‐axial material stressing. An influence on the life duration is not proved. The structure and the mechanism of their generation are under discussion. There exist phenomena of the microstructure which are similar to the white bands, the so called adiabatic shear bands. These generate after a local macro‐shear process of the microstructure, caused by a single, rapid shock‐shear stressing. Flash‐temperatures near the melting point generate in the shear zone of martensitic hardened materials the formations of new hardened zones; these microstructures cannot be etched too. The paper contributes to the question, if there exists a stress‐constellation which causes the adiabatic formation of butterflies and of white bands during the steady and long during process of plastic deformation. If one considers the latest knowledge about rolling contact fatigue and stressing by EHD (elasto‐hydro‐dynamic) flow, he can come to the conclusion, that at least the “high‐angle white bands” are adiabatic shear bands.     

Nichtrostende Stähle mit TRIP/TWIP/SBIP‐Effekt

Stainless steels with TRIP/TWIP/SBIP effect Economic austenitic steels with high energy absorption capability are in the focus of worldwide research activities, whereby the steels which show TRIP, TWIP and/or SBIP effects play a crucial role. New austenitic or austenitic‐martensitic stainless steels with a high cold workability and energy absorption capability are currently developed and tested in laboratory scale at the Institute of Iron and Steel Technology at the Technical University Bergakademie Freiberg. The mechanical properties of these steels are essentially influenced by the TRIP, TWIP and SBIP effect, becoming evident in hot formed and solution annealed steels as well as in as‐cast steels. The TRIP/TWIP/SBIP effects have a significant impact on the toughness and the strength of stainless steels consisting of metastable austenite. The TRIP effect owns a paramount position since it serves for a simultaneous increase of toughness and strength. The influences of alloying elements like manganese or nickel on the TRIP effect are in the centre of the investigations at the Institute of Iron and Steel Technology. These austenitic or austenitc‐martensitic stainless steels provide the ability for new applications fields due to their excellent mechanical properties. Exemplary, in the Collaborative Research Centre SFB 799 “TRIP‐Matrix‐Composites”, financed through the Deutsche Forschungsgemeinschaft DFG, the suitability of this new class of steels for cast components in ductile and transformation strengthened high performance (metal) ceramic composite materials will be investigated.     

Kenngrößen von Turbo‐Molekularpumpen

Graphs of “high‐vacuum pressure as a function of backing pressure” (“p_HV versus p_VV”) and “compression as a function of backing pressure” (“K versus p_VV”) are presented in this article. The performance of any turbomolecular pump can be fully and reliably evaluated with the aid of these graphs. Until now these graphs have only seldom been shown in catalogs. The catalogs generally lack the so‐called “limit lines” (“Q as a function of p_{VV, Kmax}”). For a prescribed gas throughput Q, the limit line indicates what minimum pressure must be generated by the backing pump at the fore‐line port of the turbomolecular pump so that a stable pressure exists at the high‐vacuum side of the turbomolecular pump. Using the gas‐type‐specific limit line and the corresponding, usually well‐documented pumping‐speed curve, one can already describe the functional proficiency and performance of a selected combination of turbomolecular pump plus backing pump in an approximate manner – but not yet completely. In this article we also indicate analytical functions which excellently describe the pressure dependence of the compression and pumping speed.     

Fracture strains in sheet metalforming and specific essential work of fracture

It is proposed that a plausible relationship exists between the specific essential work of fracture (R) obtained from testpieces incorporating starter cracks and the work required locally to nucleate a crack in flawless thin sheet metals. This relationship allows fracture strains in sheet metalforming to be determined from independent measurements of R instead of from in-situ strain measurements, which are usually employed in the construction of “fracture” forming limit diagrams. Some preliminary results on aluminium seem to support this proposed concept but much more work is required before a definite conclusion can be reached.