Effective cooling of work rolls in strip rolling

Abstract

An examination has been made of the effectiveness of roll cooling as applied to strip rolling, taking into account practical considerations such as the spray location and positioning, coolant flowrate, and rolling temperature. It is indicated by the results that, for cold rolling, the most effective cooling can be achieved by commencing spray cooling at the exit side of the roll gap and by providing a large spray contact angle. However, for hot rolling, cooling should commence at least 45° from the roll gap exit and a high spray flux density with relatively small spray contact angle is more appropriate.

MST/642     

Relation between work of fracture and fracture toughness of short-fibre reinforced polymers

On the basis of micromechanical failure mechanisms acting in short-fibre reinforced polymers we derive the crack resistance curve (R-rmcurve) as a function of crack extension. With the conditions characterizing the point of crack instability, the critical crack extension and fracture toughness, G_c, are calculated. By comparing G_c to the work of fracture we are able to conclude that, depending on the relevant failure characteristics, two parameters are necessary to describe the failure behaviour—the fracture toughness for crack instability or strength and the work of fracture as the energy necessary to drive the crack through the sample.     

Experimental investigation on thermal wear of high speed steel rolls in hot strip rolling

Abstract

The thermal wear ratio of a high speed steel roll was investigated experimentally in hot strip rolling with a DTW- 166 thermal wear testing machine developed by the authors. It is clear that the wear ratio increased with number of cycles. Some of the increase in wear was because of the black oxide layer generated on the roll surface at the beginning. The wear ratio also increased as slippage ratio and loads increased. Loads played a more important role than slippage ratio for thermal wear. The appearance of the roll surface was observed by SEM under different conditions. The mechanism of thermal wear was composed of adhesive, microploughing, microcutting, oxidation, and plastic slippage wear.     

Characterising fibre compression fracture toughness of composites using bearing tests

In this paper we propose the use of a bearing test with a coupled experimental–numerical approach to characterise the critical strain energy release rate, or “fracture toughness”, for fibre compression failure in bearing. This property is used in continuum damage mechanics (CDM) approaches for progressive failure analysis of composite laminates. In the proposed approach, experimental results for a standard bearing test are used to calibrate the fracture toughness with a progressive failure analysis using a CDM damage model. The approach is demonstrated for a plain weave carbon/epoxy material using the CDM damage model available in a commercial finite element package (Abaqus). The results indicate that the bearing test method provides a simple and convenient means of quantifying fibre compression fracture toughness. Analysis results applying the characterised value show good comparison with experimental results, and confirm the value of the bearing test as part of a novel material characterisation technique.     

Deformation and fracture of paper during the in-plane fracture toughness testing--Examination of the essential work of fracture method

The essential work of fracture (EWF) method is applied to various machine-made papers. The deforming and fracturing processes of the paper samples during testing is analyzed by means of the thermographic observation. Plastic deformation zone appears in three ways when deep double edge notched tension specimens are strained under in-plane stress: i.e. 1. type (i)--appearing through whole the ligament in a vague manner and developing into a circular (or oval) zone even before or at the maximum load point; 2. type (ii)--appearing from notch tip and amalgamating into a circular (or oval) zone after the maximum load point; and 3. type (iii)--appearing from notch tip and not amalgamating into a circular (or oval) zone until the sheet failure. Specimens with small ligament length (L) are likely to belong to type (i), while those with large L to type (ii) & (iii). Among these three types, type (i) fulfills the original assumption of the EWF method best in terms of the complete ligament yielding before crack initiation. Thus the specific essential work of fracture determined using the linear relation of type (i) should be correct, although the estimated work is a little smaller than that from the linear relation of type (ii) & (iii).     

Validation of single-edge V-notch diametral compression fracture toughness test for porous alumina

The objectives of this study were to validate a single-edge V-notch diametral compression fracture toughness technique for ceramics. Rounded notches and sharpened V-notches were introduced into porous, fine-grained alumina samples, and the fracture toughness results were compared. A theory linking the toughness of the material to the degree of densification fit the fracture toughness results well. The data for the V-notch samples were found to correlate very well with published results for porous alumina. The fracture toughness values were found to be independent of the specimen thickness or notch depth.     

Potential application of electron work function in analyzing fracture toughness of materials

Fracture toughness determines materials' resistance to fracture, which is measured often using impact or bending tests. However, it is difficult to evaluate fracture toughness of coatings and small samples.In this article, using white irons as sample materials, we explore a possible approach of using electron work function(EWF) as an indicator in evaluating fracture toughness of hard metallic materials. This parameter is promising for being utilized to analyze toughness of protective coatings and small objects as well as bulk materials. Through comparison with results obtained from impact tests and elastic modulus measurement, effectiveness of this EWF approach is demonstrated.     

Comparison of thermomechanical stresses produced in work rolls during hot and cold rolling of Cartridge Brass 1101

Thermomechanical stresses play an important role in defining the life of the work roll used in hot rolling process. In this research temperature dependent mechanical properties of cartridge brass are determined experimentally using high temperature compression tests at different temperatures and strain rates. Real life measurements are made from a brass rolling mill as input data for the simulation boundary conditions. Hot rolls are made of AISI H11 hot work tool steel. Temperature dependent mechanical properties of AISI H11 steel are used. Thermal and mechanical stresses produced in the work rolls during hot rolling process are predicted using a thermoplastic finite element approach in the ABAQUS Standard software. Hot rolling is compared with cold rolling to determine the effects produced on the work rolls. A criterion is introduced to compare the severity of stresses produced on the rolling surfaces in case of hot rolling and cold rolling based on the yield stress of the roller material for different temperatures. A method for separating thermal and mechanical stresses in the simulation is also described.     

The fracture toughness of borides formed on boronized cold work tool steels

In this study, the fracture toughness of boride layers of two borided cold work tool steels have been investigated. Boriding was carried out in a salt bath consisting of borax, boric acid, ferro-silicon and aluminum. Boriding was performed at 850 and 950 °C for 2 to 7 h. The presence of boride phases were determined by X-ray diffraction (XRD) analysis. Hardness and fracture toughness of borides were measured via Vickers indenter. Increasing of boriding time and temperature leads to reduction of fracture toughness of borides. Metallographic examination showed that boride layer formed on cold work tool steels was compact and smooth.     

Influence of heat treatment and hot working on fracture toughness of cast aluminium base composites

Abstract

Evaluation of toughness in terms of the fracture energy E*, obtained using Charpy impact testing and the fracture toughness K_Ic obtained from bend tested specimens, has been carried out for various cast particle reinforced aluminium base composites, namely, A356–SiC, A357–SiC, 6061–Al₂O₃, and 2014–Al₂O₃. In practice, the first two are used in the as cast foundry condition and the last two in the cast and extruded condition. Hot extrusion or rolling to reduction ratios between 2 : 1 and 50: 1 was conducted on the 6061 and 2014 composites to characterise the influence of working processes. Heat treatment conditions considered included the as cast (or as worked), solid solution treated, and T6 temper. The results show that extrusion or rolling can markedly improve the toughness, but on thermal aging the toughness is reduced. The increase in total fracture energy by hot working is mainly caused by the increase of initiation energy, whereas the decrease of fracture energy by artificial aging is controlled by the propagation energy. The values of K_Ic obtained for these composites are from 15 to 25 MN m^?3/2. Comparisons and interpretations of the dynamic Charpy fracture energy, quasistatic fracture toughness, and fracture surface of the four composites are also presented.

MST/1806     

Essential work of fracture compared to fracture mechanics—towards a thickness independent plane stress toughness

The essential work of fracture (EWF) and the J-integral methods were applied in a study of the effect of the thickness on the cracking resistance of thin plates. The paper discusses two themes: (1) the relationships between the two methods or concepts is elucidated, and (2) a new, thickness independent plane stress toughness parameter is proposed. For that purpose, cracked aluminium 6082O thin plates of 1-6 mm thickness were tested in tension until final separation. The EWF, w_e, and the J-integral at cracking initiation, J_i, increase identically with thickness except at larger thickness for which the increase of J_i levels off. J_i reaches a maximum for 5-6 mm thickness whereas w_e keeps increasing linearly with thickness. This difference is related to the more progressive development of the necking zone in front of the crack tip when thickness increases: at large thickness, cracking initiates well before the neck has developed to its stationary value during propagation. A linear regression on the fracture toughness/thickness curve allows partitioning the two contributions of the work of fracture: the plastic work per unit area for crack tip necking and a plane stress work per unit area for material separation. The pertinence of this new measure of the pure plane stress cracking resistance is critically discussed based on a micromechanical model for ductile fracture. The micromechanical void growth model incorporates void shape effects, which is essential in the low stress triaxiality regime.     

Measuring toughness and the cohesive stress-displacement relationship by the essential work of fracture concept

The complete fracture behaviour of ductile double edge notched tension (DENT) specimen is analysed with an approximate model, which is then used to discuss the essential work of fracture (EWF) concept. The model results are compared with the experimental results for an aluminium alloy 6082-O. The restrictions on the ligament size for valid application of the EWF method are discussed with the aid of the model. The model is used to suggest an improved method of obtaining the cohesive stress-displacement relationship for the fracture process zone (FPZ).     

Evaluation of fracture toughness of ABS polymers via the essential work of fracture (EWF) method

The essential work of fracture (EWF) method was employed to determine the fracture toughness of SAN/PB-g-SAN blends with the compositions of 65/35–0/100. It was found that the plane stress EWF approach is applicable for different SAN/PB-g-SAN blends. During EWF tests, three different types of load–displacement curves were recorded, depending on the blend composition. For the samples containing rubbery phase of 35–65 wt% crack growth occurred immediately after full ligament yielding. Further increase in rubber content and for the samples with 75 and 85 wt% of rubbery phase, the onset of crack growth was preceded by the formation of necking zone in the form of a sharp load drop after full ligament yielding. For the sample with the composition of 0/100, strain hardening behavior was observed without any sign of neck formation. The specific essential work of fracture (EWF) w _e increased at first with the increase of PB-g-SAN wt% and then decreased with further increasing amount of rubbery phase. The maximum value of w _e was observed for the blend with 75 wt% of PB-g-SAN. The specific non-essential work of fracture βW _p increased with increasing of rubbery phase concentration. The analyzing of yielding and necking/tearing components of essential and non-essential parameters showed that for the samples containing 55 wt% and higher of rubbery phase, w_\texte,nt \succ w_\texte,y w_{\text{e,nt}} \succ w_{\text{e,y}} and b_\textnt w_\textp,nt \succ b_\texty w_\textp,y \beta_{\text{nt}} w_{\text{p,nt}} \succ \beta_{\text{y}} w_{\text{p,y}} , indicating that a majority of fracture energy was dissipated in the necking and tearing stages of fracture process. Finally, it is shown that w _e values can be predicted via COD values.     

Effect of silicon on the fracture toughness and oxidation behavior of hot pressed NbCr2 alloys

NbCr₂ Laves phase alloyed with 0–7 wt.% Si was fabricated by mechanical alloying followed by hot pressing. The influence of silicon on the mechanical properties and oxidation behavior of NbCr₂ were investigated. It was revealed that Si addition has a beneficial effect on the oxidation resistance and fracture toughness of NbCr₂ alloy. The addition of Si partially occupies the Cr site in the Laves phase and partially forms the hard Nb₅Si₃ phase, which can yield an increase in the hardness of as-HPed NbCr₂ alloys. When alloying with 5 wt.% silicon, the fracture toughness value of NbCr₂ reaches the highest (6.45 MPa √m) which is about 13% more than that of unalloyed NbCr₂ and is 4 times higher than that of cast materials (1.2 MPa √m). Addition of silicon also resulted in a substantial improvement in the oxidation resistance of the NbCr₂ alloys exposed in air at 1373 K and 1473 K.     

Oxidation behaviour of ledeburitic steels for hot rolls

The oxidation behaviour of two high speed steels (HSS) employed for the production of hot rolls was studied. The steels have slightly different chromium contents and volume fractions of primary carbides. Because oxidation nucleates at the matrix–carbide interfaces and propagates in the matrix without involving the carbides, the oxide scale grows less uniform. All the primary carbides have a higher Cr content than the matrix; therefore they tend to reduce the oxidation resistance. The slight differences in chromium content and in carbide volume fraction are responsible for the different oxidation resistance at 600 and 700°C, whilst at 500°C the two steels have almost the same resistance.     

The fracture toughness and fracture morphology of polyester resins

The change in fracture toughness of an orthophthalate based medium reactivity polyester resin with change in resin flexibility, catalyst content, cure temperature and liquid environment, has been investigated. Various specimen geometries including simple double cantilever beam and tapered double cantilever beam have been used and the results obtained with the different geometries are in good agreement. The critical flaw size for the resin has been determined and an estimate obtained of the radius of the plastic zone under plane strain conditions. The solvent absorption characteristics of the resin were found to be effected by catalyst content, resin flexibility and by residual strains. Observations of the fracture behaviour, using both fracture toughness and tensile specimens have revealed a fracture surface morphology very similar to that of glass but with limited plastic deformation occurring.     

Fatigue fracture toughness of metals

The paper considers the peculiarities of fatigue crack propagation and final fracture of metals under cyclic loading. It is shown that the value of the fatigue fracture toughness of steels in an embrittled state is appreciably lower than that of the fracture toughness under static loading. A model of the transition from stable to unstable fatigue crack propagation is justified.