Transformation Conditions ‐ Microstructures ‐ Mechanical Properties Relationship in 0.60%C Hypoeutectoid Steel

The relationship between transformation conditions, microstructures and mechanical properties were investigated on a 0.60%C hypoeutectoid steel, which has predominantly pearlitic microstructure. The steel had been austenized at 850‐1050 °C and then isothermally transformed at 570‐660 °C. Afterwards, the microstructures were analyzed by optical microscope, scanning electron microscope, and quantitative metallography. And the mechanical properties were determined. The results indicated that the interlamellar spacing, the pearlitic colony size and the proeutectoid ferrite content follow a linear relationship with the reciprocal of the undercooling, and the strength, the hardness and the toughness follow a Hall‐Petch type of relationship with the inverse of the square root of the interlamellar spacing. Besides, the stress condition should be taken into account during analyzing the effect of the interlamellar spacing on the mechanical properties. The results have been explained on the basis of transformation conditions ‐ microsturctures ‐ mechanical properties ralationship.     

An Assessment of Vacuum‐Heat‐Treated H11 Hot‐Work Tool Steel using the KIc/HRc Ratio

Charpy V‐notch (CVN) impact‐test values are widely used in toughness specifications for AISI H11 hot‐work tool steel, even though the fracturing energy is not directly related to the tool design. K_Ic, the plain‐strain stress‐intensity factor at the onset of unstable crack growth, can be related to the tool design; however, K_Ic test values are not widely used in toughness specifications. This is surprising since to the designer K_Ic values are more useful than CVN values because the design calculations for tools and dies of high‐strength steels should take into account the strength and the toughness of materials in order to prevent the possibility of rapid and brittle fracture. An investigation was conducted to determine whether standardized fracture‐toughness testing (ASTM E399‐90), which is difficult to perform reliably for hard materials with a low ductility, could be replaced with a so far non‐standard testing method. A particular problem is that the manufacture of the fatigue crack samples is difficult and expensive, and this has promoted the search for alternative fracture‐toughness testing methods. One of the most promising methods is the use of circumferentially notched and fatigue‐precracked tensile specimens. With this technique the fatigue crack in the specimen is obtained without affecting the fracture toughness of the steel, if it is obtained in soft annealed steel, i.e., prior to the final heat treatment. The results of this investigation have shown that using the proposed method it was possible to draw, for the normally used range of working hardness, combined tempering diagrams (Rockwell‐C hardness ‐ Fracture toughness K_Ic ‐ Tempering temperature) for some AISI H11 hot‐work tool steel delivered from three steel plants. On the basis of the combined tempering charts the influence of the processing route on the mechanical properties was investigated. In the same way, vacuum‐heat‐treated tool steels were assessed and their properties expressed as a ratio of the fracture toughness to the hardness (K_Ic/HRc).     

Characterization of Steel Thixoforming Tool Materials by High Temperature Compression Tests

The experimental set‐up and the results of high temperature compression tests for the characterization of tool materials for steel thixoforming are presented. The scope of this test is to reproduce the load profile of steel thixoforming processes consisting of mechanical, thermal, tribological, and chemical components on the forming mould. Tool materials were chosen following a concept within the Collaborative Research Center ‐ SFB 289 ‐ “Forming of metals in the semi‐solid state and their properties”. Three materials groups are distinguished: thin film deposited by physical vapor deposition (PVD) and plasma assisted chemical vapour deposition (PACVD), thick coatings (thermal spraying), and bulk ceramic materials. Samples were characterized using scanning electron microscopy (SEM), electron‐dispersive spectroscopy (EDS) and X‐Ray diffraction analysis (XRD). The results show varying resistance of the tool materials concerning the load profile. In order to provide an appropriate tool solution for the thixoforming of steels, different load profiles within the forming moulds are identified and the corresponding tool part is made from that material with the best performance.     

Assessment of the Primary Structure of Slabs and the Influence on Hot‐ and Cold‐Rolled Strip Structure

The quality of the delivered strips depends ‐ among other things ‐ on the internal structure of the slabs. Solidification phenomena as micro‐ and macro‐segregation are of particular importance. This paper deals with the micro‐ and macro‐segregations in slabs of the steel grades DP and S355. The method for the investigation of segregations based on electron probe microanalysis is described in brief. Characteristic values of segregations are derived from quantitative element distribution images. These results serve as a basis for decisions on further processing steps in the hot and cold rolling mill.     

Microstructures and Mechanical Properties of High‐Strength Fe‐Mn‐Al‐C Light‐Weight TRIPLEX Steels

High‐strength TRIPLEX light‐weight steels of the generic composition Fe‐xMn‐yAl‐zC contain 18 ‐ 28 % manganese, 9 ‐ 12 % aluminium, and 0.7 ‐ 1.2 % C (in mass %). The microstructure is composed of an austenitic γ‐Fe(Mn, Al, C) solid solution matrix possessing a fine dispersion of nano size κ‐carbides (Fe,Mn)₃ AlC_1‐x and α‐Fe(Al, Mn) ferrite of varying volume fractions. The calculated Gibbs free energy of the phase transformation γ_fcc → ?_hcp amounts to ΔG^γ→? = 1757 J/mol and the stacking fault energy was determined to Γ_SF = 110 mJ/m². This indicates that the austenite is very stable and no strain induced ?‐martensite will be formed. Mechanical twinning is almost inhibited during plastic deformation. The TRIPLEX steels exhibit low density of 6.5 to 7 g/cm³ and superior mechanical properties, such as high strength of 700 to 1100 MPa and total elongations up to 60 % and more. The specific energy absorption achieved at high strain rates of 10³ s^?1 is about 0.43 J/mm³. TEM investigations revealed clearly that homogeneous shear band formation accompanied by dislocation glide occurred in deformed tensile samples. The dominant deformation mechanism of these steels is shear band induced plasticity ‐SIP effect‐ sustained by the uniform arrangement of nano size κ‐carbides coherent to the austenitic matrix. The high flow stresses and tensile strengths are caused by effective solid solution hardening and superimposed dispersion strengthening.     

The Microstructure and Mechanical Properties of Ultrafine Grained Plain C‐Mn Steels

An ultrafine microstructure was produced in plain C‐Mn steels with different carbon contents (0.15 ‐ 0.3 mass% C) by heavy warm deformation. The rolling was simulated by the plane strain compression test with a simulated post rolling coiling. The final microstructure consists of an ultrafine grained ferrite matrix with the average grain size of 1.1 ‐ 1.4 μm and spheroidized cementite particles of two different size groups. The fraction of high‐angle grain boundaries maintained in the range of 60% to 65%. With the increase of C content from 0.15 mass% to 0.3 mass% the strength increases by about 100 MPa, while the total elongation of 23% hardly changes. The (specific) upper shelf energy decreases from 320 J/cm² to 236 J/cm² but a rather low ductile‐to‐brittle transition temperature (DBTT) of about 206 K does not rise with increasing C content. The ultrafine steel with higher C content (0.3 mass%) exhibits a superior strength‐toughness combination.     

The role of surface phenomena in powder metallurgy processes— A review

Conclusions A survey is presented of the role of surface phenomena at all stages of formation of sintered bodies by the methods of powder metallurgy.The properties of powders are shown to be determined chiefly by the surface properties. The role of surface phenomena in obtaining metal powder by various methods (reduction, electrolytic, mechanical, dispersion) is discussed in the paper. Surface phenomena are also important in pressing processes, and are a motive force in sintering processes. This is distinctly seen on sintering in the presence of the liquid phase and in impregnation processes. An attempt is made to link the strength of binding and microstructure in cermets with the surface properties of their components.     

Microstructural and mechanical properties investigation of electrodeposited and annealed LIGA nickel structures

Lithographic, Galvanoformung, Abformung (LIGA) component fabrication is a process in which structural material is deposited into a patterned polymethyl-methacrylate (PMMA) mold realized through deep X-ray lithography. The process permits fabrication of metal microelectrome chanical systems (MEMS) components with representative dimensions that range from a few microns to several millimeters. This investigation characterizes the microstructure and mechanical properties of LIGA-fabricated nickel (LIGA Ni), electrodeposited using Watts bath and sulfamate bath chemistries. As a prelude to studying high-temperature joining processes in LIGA Ni components, an annealing investigation was conducted on samples fabricated from both bath chemistries. Mechanical properties and microstructural analyses on as-deposited and annealed samples were conducted using a mini servohydraulic load frame and the electron backscatter diffraction (EBSD) microtexture measurement technique. The deposits were found to have fine-grain, highly textured microstructures oriented with an acicular or columnar morphology relative to the plating direction. Previously uncharacterized, anomalous, local spatial variations in the crystallographic texture of the as-deposited microstructures were identified by EBSD analyses. Microstructural evolution during annealing seemed to follow a recovery, recrystallization, rapid grain-growth microstructural-evolution mechanism in LIGA Ni deposited from the Sulfamate bath chemistry and simply a recovery and grain-growth microstructural-evolution mechanism in LIGA Ni deposited from the Watts bath chemistry. The evolution of microstructure in the annealed samples corresponded with a dramatic drop in their strength and determined the limiting diffusion-bonding temperature for LIGA Ni components.     

Dilatometric Studies on Copper Added Titanium‐Boron Steels

An attempt has been made to examine the effect of Cu addition in low carbon Ti ‐ B microalloyed steel on their continuous cooling transformation behaviour by dilatometric study. It has been demonstrated that addition of Cu by an amount of 1.5 wt% in Ti – B microalloyed steel effectively lowers the transformation start temperature. Addition of Ni (0.79 wt%) in 1.5 wt% Cu‐added Ti–B microalloyed samples further lowers the transformation temperature of austenite even at cooling rates comparable with air ‐ cooling condition. The microstructural investigation of the samples subjected to the dilatometric study under different cooling rates suggests the possibility of obtaining pearlite free multiphase microstructures in 1.5 wt% Cu – and 0.79 wt% Ni – added Ti – B microalloyed steel by air – cooling from the austenite region.     

Fundamentals of the Deep Rolling of Compressor Blades for Turbo Aircraft Engines

The weakening of the fatigue strength of turbine blades due to local impact damage caused by foreign objects (Foreign Object Damage ‐ FOD) represents a significant safety risk in modern civil and military aviation. It is possible to employ deep rolling to counteract such component weakening in a particularly effective way. Although deep rolling has long been used on rotationally symmetric components, the underlying deformation processes are largely unknown. The deep rolling of thin‐walled free‐form surfaces, such as those on turbine blades, represents a new, demanding production challenge. For this reason, this paper describes the implementation of a deep‐rolling process for machining thin‐walled turbine blades and, by combining practical and numerical test results, outlines some of the underlying deformation processes.     

Evaluation on some important force models for cold strip rolling

To improve the accuracy of rolling force prediction, some important force models were evaluated through applied computation for cold rolling of low carbon steel and aluminum alloy according to measured data on lab mill. The effects of model structure and three important variables ‐ flow stress, contact length and friction coefficient ‐ on the precision of computed force were quantitatively studied. Flow stress was measured with plane‐strain compression test, contact length was based on elastic flattening of work‐roll by Hitchcock, and friction‐coefficient was determined by rolling strain and numerical iteration. In steel rolling Bland & Ford integration model and Bryant & Osborn algebraic equation are better in accuracy than Ekelund and Parkins. In aluminum rolling all the models produce large deviations ΔF_R = 10–20% if flow stress, contact length and friction coefficient are determined with the same method as steel rolling. The elastic deformation of aluminum strip is now taken into account for its low elastic modulus. An effective method to determine plastic and elastic contact has been developed in this investigation. The accuracy of force computation is obviously improved for aluminum rolling.     

Microstructure Formation in AlSi7Mg Alloys Directionally Solidified in a Rotating Magnetic Field

To produce high stressed automotive components like engine frames and cylinder heads in foundry industry often AlSi7Mg alloys are used. During mould filling and casting melt flow affects the development of the microstructure, which defines the mechanical properties. In this paper the microstructure formation in AlSi7Mg0.3 and AlSi7Mg0.6 alloys during directional solidification is investigated. To induce a forced melt flow a rotating magnetic field is applied. For that purpose a Bridgman‐type gradient furnace is equipped with a rotary ring magnet. For detailed investigation of the shape of the solid‐liquid interface and the primary dendrite spacing a decanting device is used. As a result, the forced melt flow substantially changes the dendritic solidification microstructure. The rotating magnetic field generates a radial secondary flow in and ahead of the mushy zone, which causes an enrichment of eutectics in the centre of the samples. At lower solidification velocities this locally leads to the transition to mixed columnar‐equiaxed or even to equiaxed growth. In that case the solid‐liquid interfaces of the decanted samples show a significant depression in the centre part. In the out‐of‐centre region columnar growth still exists and the primary dendrite spacing decreases with increasing melt flow.     

Thermo‐elastic Homogenization of 3‐D Steel Microstructure Simulated by the Phase‐field Method

A multi‐scale approach based on the asymptotic homogenization method of periodic material structures is applied here to determine the effective thermo‐elastic properties of 3D steel microstructures, which have been calculated by phase‐field simulations. A multiphase‐field model, coupled to thermodynamic databases, is used to evaluate the microstructure evolution during the austenite to ferrite phase transformation of low carbon Fe‐C‐Mn steel. In order to derive effective mechanical properties, geometrical information about the grains, their phase properties and crystallographic orientations are transferred to the homogenization tool. Effective cubic Young and shear modules and Poisson coefficients are predicted for different ferrite volume fractions. Moreover, the volume change is derived as function of the phase fractions, leading to a calculated dilatometer curve. The effects of the thermal shrinkage and the volume expansion caused by the phase transformation are taken into account.     

Effect of Microstructure on Mechanical Properties of a Thin‐walled Cast Duplex Steel

The paper investigates the microstructure and the resulting mechanical properties of a duplex steel cast in sand dead‐moulds. The chemical composition and the cooling rate are the main parameters affecting the properties. The chemical composition influences the thermodynamics of the phase transformation, the cooling rate determines the kinetics of formation of the microstructure. The latter varies with changes in wall thickness (investigated from 2 to 7 mm) and the position of the material within the casting. Through heat treatment, the composition of the microstructural components can be changed selectively afterwards, thereby the properties are improved. The correlation between microstructure and mechanical properties is explained quantitatively. Additionally to the austenite‐ferrite ratio, the dispersion of microstructure has a large influence on the mechanical properties. Ranges of chemical composition and heat treatment parameters are identified where third phases, such as carbides and σ‐phase tend to occur. These influence the properties very sensitively even in small amounts. Finally, structural parameters are recognized that will lead to optimal combinations of properties. With an appropriate heat treatment technique, in particular the ductility properties are further increased.     

High Strength 3D Non‐Vacuum Electron Beam Weld Joints ‐ Setting of Gradient Material Properties and Testing of Weld Quality

Highly loaded structures made of modern metallic materials are increasingly being placed under stringent standards with regard to the mechanical strength and deformation properties of their individual parts as well as with regard to the loading capacity of their joints. In order to create strain profiles in individual structural components, such components are heated with an electron beam at defined locations in order to bring in local microstructure changes and consequently the targeted local changes in the strength and deformation properties of the material with respect to a delayed crack growth. Additionally, components with specifically set tensile strength are welded to high quality structures. For this purpose the young but efficient non‐vacuum electron beam welding (NV‐EBW) method is used and further developed. Weld quality is examined especially in regard to an improved beam positioning, process control and weld joint defect detection.     

Characterization of Gas Metal Arc Welded Hot Rolled DP600 Steel

Dual‐phase (DP) steels are suitable candidates for automotive applications due to their high strength and ductility. These advanced mechanical properties result from the special microstructure of the DP steel with 5～20% martensite phase in a soft ferrite matrix. However, during welding, which is an important process in automotive industry, this special microstructure is destroyed. In this research the characterization of Gas Metal Arc (GMA) welded joining zones was performed by optical microscopy and hardness mapping. Tensile tests were also performed keeping the welded portion in the gauge length. Scanning Electron Microscopy (SEM) was used for the fracture investigation. From the characterization and tensile tests, the soften zones were found, which are caused by the tempered martensite and larger ferrite grain size than that in base metal. Furthermore, GMA welding make a large Heat Affected Zone (HAZ).     

Advanced Processing and Properties of Thixotropic Formed Components Based on Partially Melted Magnesium

Contrary to the manufacture of aluminium components in the semi‐solid, thixotropic state the production of magnesium based components by semi‐solid techniques is still uncommon. For this reason, the advantage of this production method is analysed with regard to the commercial magnesium alloy AZ80. The objective of this research is semi‐solid‐casting (SSC) of AZ80 for the production of a light weight component in near‐net‐shape quality and with advanced properties. Using extruded primary feedstock material, the behaviour and the advantages are investigated. Billets with a weight of up to 2 kg are heated up into a semi‐solid state. To avoid any risk of self‐ignition of the material an automatic, temperature controlled induction heating system is used. To achieve an optimum homogeneous grain structure the induction heating power is varied making use of a process control system based on power‐time‐curves. The heated billets are transported in the soft semi‐solid condition from the induction heating system to a die casting machine to produce components with wall thickness’ between 2 and 10 mm. After forming of the components, the influence of heat treatment on the grain structure and especially on the mechanical properties is determined to provide parts with optimised characteristics. To compare the properties of the special globular grain and microstructure, the results of various static and dynamic tests are analysed. It is found that components can be manufactured with a magnesium alloy in a thixotropic state in near‐net‐shape quality, with low porosity and with excellent mechanical properties like elongation of up to 15%.     

Norican Steel ‐ An Assessment of the Archaeological Finds at the Magdalensberg Site,Carinthia, Compared to the “Celtic Trove” of Gründberg Hill,Linz

The analysis of Celtic steel products from depots found in the area north of the Danube has shown that their excellent properties are due to the choice of unalloyed or alloyed steel bars that were shaped by fire‐forging. Phosphorus was used as an alloying element, while carbon was added through carburizing of the workpiece. Was this forging technique also known to regions in the very heart of the Alps? Our analysis set out to find an answer to this question. The Celtic smiths in the inner Alpine regions used shaft furnaces to produce steel loops with numerous slag inclusions but very low contents of trace elements in the steel matrix. In a series of process steps in the smithy ‐ trimming the steel loop, forging into bars, “sheafing”, carburizing, hardening ‐ the steel properties demanded by the customers were achieved. Phosphorus was not deliberately employed as an alloying metal in the valleys of the Central Alps. The fact that the forging technique was tailored to the actual product appears to underlie the high quality of “Norican steel”.     

Influence of heat treatment parameters on structure and mechanical properties of an HSLA‐100 steel

An HSLA‐100 steel received from the US Naval Research Laboratory has been characterised. The effects of heat treatment parameters such as austenitisation time and temperature, tempering time and temperature on mechanical properties have been studied. The microstructures resulting from different heat treatment conditions have been correlated with mechanical properties through SEM and TEM studies. Quantitative relationships have been developed between mechanical properties and the operational variables within a narrow range of variation of the variables by statistical design of experiments. A quantitative relationship has also been developed for the same for a wider experimental region through curve fitting technique. The best combination of strength and low‐temperature toughness was obtained in the region of 700 °C tempering temperature and 0.3 ‐ 0.4 h tempering time.     

Detailed study of the transformation mechanisms in ferrous TRIP aided steels

Development of TRIP aided ferrous alloys is one answer to the demand for weight decrease in the automotive industry. The microstructure of hot rolled and cold rolled TRIP steels is quite complex and the optimisation of such steel products requires a detailed understanding of the mechanisms of phase transformation, during thermomechanical treatment as well as during mechanical testing or metal forming. We present in this paper the results obtained at Irsid concerning the study of austenite stabilisation through bainitic transformation during thermal treatment and its transformation into martensite during mechanical testing. First of all, the characterisation methods are presented. An effort has to be put on this point due to the refinement of the microstructure of TRIP steels, especially the size of austenite and martensite islands. Carbon replicas for the observation by means of transmission electron microscopy (TEM) are used to analyse the morphological features of the microstructure ‐ nature of the constituents, size and shape ‐ and the composition of cementite particles present in the steels. The mean value for this carbon content in retained austenite is deduced from X‐ray diffraction measurements. Then the kinetics of bainitic transformation are discussed as well as cementite precipitation. The typical composition of the steel studied is 0.5 % C, 1.5 % Mn. The use of 0.5 % C steels facilitates the study of bainitic transformation by avoiding the ferrite formation usually occurring in TRIP steels. Cementite nucleation appears at the ferrite/austenite interface without any partitionning of substitutional elements. To satisfy thermodynamic equilibrium at the interface, the silicon content on the cementite side is very low and high on the austenite side. Then, carbon diffusion towards austenite is delayed and, as a consequence, cementite growth is also delayed. As the diffusion kinetics are low at 400 °C, cementite keeps this “non partitioned” composition, even after 3 hours holding. At 500 °C, diffusion kinetics are higher and cementite composition approaches that predicted by equilibrium. Finally, the stability of retained austenite during mechanical testing is studied. Before and after mechanical testing the morphological characteristics of the microstructure (austenite island size and elongation) are analysed by TEM replicas and image analysis. There is a high density of very small austenite islands but they represent only a small fraction of the total retained austenite. These results confirm and quantify the size effect on austenite stabilisation during deformation.