The effect of heat treatment on the gouging abrasion resistance of alloy white cast irons

A series of heat treatments was employed to vary the microstructure of four commercially important alloy white cast irons, the wear resistance of which was then assessed by the ASTM jaw-crusher gouging abrasion test. Compared with the as-cast condition, standard austenitizing treatments produced a substantial increase in hardness, a marked decrease in the retained aus-tenite content in the matrix, and, in general, a significant improvement in gouging abrasion resistance. The gouging abrasion resistance tended to decline with increasing austenitizing tem-perature, although the changes in hardness and retained austenite content varied, depending on alloy composition. Subcritical heat treatment at 500 ° following hardening reduced the retained austenite content to values less than 10 pct, and in three of the alloys it caused a significant fall in both hardness and gouging abrasion resistance. The net result of the heat treatments was the development of optimal gouging abrasion resistance at intermediate levels of retained aus-tenite. The differing responses of the alloys to both high-temperature austenitizing treatments and to subcritical heat treatments at 500 ° were related to the effects of the differing carbon and alloying-element concentrations on changes in theM _s temperature and secondary carbide precipitation.     

The influence of heat treatment on the high-stress abrasion resistance and fracture toughness of alloy white cast irons

The influence of a range of austenitizing and subcritical (tempering) heat treatments on the high-stress abrasion resistance and fracture toughness of four commercially significant grades of alloy white cast iron was investigated. Complementing an earlier study^[1] on the influence of a more limited range of heat treatments on the gouging abrasion performance of the same alloys, the results showed that the effect of austenitizing temperature on high-stress abrasion pin test weight loss differed for each alloy. With increasing austenitizing temperature, these results ranged from a substantial improvement in wear performance and retention of hardness through to vir-tually no change in wear performance and substantial falls in hardness. Fracture toughness, however, increased markedly in all alloys with increasing austenitizing temperature. Tempering treatments in the range 400 °C to 600 °C, following hardening at the austenitizing temperature used commonly in industrial practice for each alloy, produced significant changes in both hard-ness and wear performance, but negligible changes in fracture toughness. Most importantly, the data showed that selection of the correct temperature for subcritical heat treatment to reduce the retained austenite content for applications involving repeated impact loading is critical if abrasion resistance is not to suffer.     

On thermal shock resistance of austenitic cast irons

This research applied rapid induction heating and water quenching cycles to study the thermal shock resistance of high-nickel austenitic cast irons. Both flake graphite (FG) and compacted graphite (CG) cast irons were evaluated. In addition, alloying of cobalt and (Cr + Al) to the high-nickel base materials was performed to produce ultralow thermal expansion and hightemperature specialty irons. Microstructural analysis, as well as mechanical and thermal properties testings, was performed to find out the relationship among microstructures, properties, and thermal shock resistance of the materials under investigation. LIN-CHAO WENG, formerly Graduate Student, Department of Materials Engineering, Tatung Institute of Technology.     

低碳贝氏体钢低温CTOD试验断口分离分析

通过对低碳贝氏体钢在-20℃CTOD试验中出现的分离断口进行微观分析,研究了断口分离和失稳断裂的原因.结果表明,断口分离面均为脆性断裂,至主断面为韧性扩展,分离裂纹产生的主要因素不是夹杂物,而是材料内部的带状组织和成带状的硬相组织受三维应力作用的结果.拉伸材料的分离裂纹是在达到一定抗拉强度之后开始萌生和扩展,不影响材料的使用性能.在断裂韧度试验中,一方面分离裂纹能降低材料裂纹尖端的三维应力约束,提高材料的韧性;另一方面较大程度的分离裂纹,减小了裂纹形核功和扩展功,诱发主裂纹的失稳扩展.     

Solidification structure and abrasion resistance of high chromium white irons

Superior abrasive wear resistance, combined with relatively low production costs, makes high Cr white cast irons (WCIs) particularly attractive for applications in the grinding, milling, and pumping apparatus used to process hard materials. Hypoeutectic, eutectic, and hypereutectic cast iron compositions, containing either 15 or 26 wt pct chromium, were studied with respect to the macrostructural transitions of the castings, solidification paths, and resulting microstructures when poured with varying superheats. Completely equiaxed macrostructures were produced in thick section castings with slightly hypereutectic compositions. High-stress abrasive wear tests were then performed on the various alloys to examine the influence of both macrostructure and microstructure on wear resistance. Results indicated that the alloys with a primarily austenitic matrix had a higher abrasion resistance than similar alloys with a pearlitic/bainitic matrix. Improvement in abrasion resistance was partially attributed to the ability of the austenite to transform to martensite at the wear surface during the abrasion process.     

Optimizing fracture toughness and abrasion resistance in white cast irons

A series of twelve Cr-Mo white irons varying in carbide volume from 7 to 45 pct were tested for dynamic fracture toughness and wet sand abrasion resistance. Carbon content was varied from 1.4 to 3.9 pct. Two matrix microstructures were employed, and the compositions (copper and chromium content) were varied to assure constant matrix compositions. Chromium was varied from 11.6 to 25.7 pct. In addition, one composition of white iron was subjected to thirty different heat treatments to define the effect of matrix microstructure on dynamic fracture toughness and abrasion resistance. It was shown that for the abrasive wear system used, a carbide volume of about 30 pct represented an optimum quantity, above which abrasion resistance decreased. Martensitic irons provided consistently better abrasion resistance than austenitic irons. Dynamic fracture toughness decreased with carbide volume, as expected. Higher toughness values were obtained with predominantly austenitic matrix microstructures than with predominantly martensitic matrix microstructures. Considering both abrasion resistance and fracture toughness, it was shown that heat treated irons could provide an optimal combination of these properties. Formerly Visiting Research Metallurgist, Climax Molybdenum Co. Research Laboratory.     

Fracture resistance of various carburized steels

Fracture toughness and fatigue studies were conducted on various grades of car-burizing steels. The results indicate that similar or superior mechanical performance can be achieved using the alternate EX grades (EX24, EX29, and EX55) compared to the more commonly used grades (SAE 8620, SAE 4320, and SAE 4817). During the study of fatigue properties, various trends on the influence of surface carbon (re-tained austenite) and carbon gradient (hardness profile and case depth) were observed: 1) increasing the surface carbon to the extent that the hardness profile indicated a max-imum at 0.02 in. (0.5 mm) below the surface severely reduced the endurance limit, 2) changing the hardness profile such that the maximum hardness occurred within 0.010 in. (0.25 mm) of the surface improved the endurance limit, but lowered the impact fracture stress, and 3) increasing the case depth improved the endurance limit. This paper is based on a presentation made at a symposium on “Carburizing and Nitriding: Fundamentals, Processes and Properties” held at the Cincinnati Meeting of The Metallurgical Society of AIME, November 11 and 12, 1975 under the sponsorship of the Heat Treatment Committee.     

Fracture resistance of complex amalgam restorations

We surveyed the clinical presentation, initial management and subsequent course of a prospectively registered cohort of 60 children with insulin-dependent diabetes mellitus (IDDM) diagnosed before age 15 years in the Sultanate of Oman between January 1990 and December 1993. Clinical details from the time of diagnosis were available on all the children. At diagnosis 9 (15 per cent) presented with severe ketoacidosis (DKA) with pH less than 7.1 or plasma bicarbonate less than 10 mmol/l, and 16 (27 per cent) had mild to moderate ketoacidosis with pH 7.1-7.35 or plasma bicarbonate 10-18 mmol/l. During DKA electrolyte disturbances included: hypokalemia (K < 3.5 mmol/l) 25 per cent), hyperkalemia (K > 5.5 mmol/l) (18 per cent) and hyponatremia (Na < 130 mmol/l) (40 per cent). Serum creatinine concentrations were high in 25 per cent of children with DKA. Within the first year of diagnosis, 17 of the 60 children (28 per cent) experienced symptomatic hypoglycaemia, which in six (10 per cent) led to one or more admissions. Re-admission for unstable glycaemic control, excluding acute hypoglycaemia occurred at least once in six children (10 per cent) within 1 year of diagnosis and in 10 (17 per cent) within 2 years. Statural growth velocity (GV) and GVSDS (6.9 +/- 0.85 cm/year and 0.75, respectively) were significantly higher in the group of children with good glycaemic control (HbA1C = 7.9 +/- 0.4 per cent) compared to those children (3.7 +/- 0.44 cm/ year and -1.6, respectively) with bad glycaemic control (HbA1C = 12.5 +/- 1.5 per cent). Insulin-like growth factor-I (IGF-I) concentrations were significantly higher (260 +/- 21 ng/ml) in the group with good glycemic control v. the group with bad control (149 +/- 15 ng/ml). In summary, greater public and medical awareness of the presenting features of diabetes in young children is needed to reduce the frequency of DKA at presentation, and improvement of patient and family education is necessary to reduce the incidence of DKA and hypoglycaemia in children with IDDM.     

Controlled graphitization as a potential option for improving wear resistance of unalloyed white irons

Effect of heat treatment on the microstructure and resistance to abrasive wear has been studied in an unalloyed white iron used for manufacturing cylindrical pebbles used as grinding media by the cement and other industries. Heat treatment comprised holding at 800 °C, 850 °C, 900 °C, and 950 °C for 30, 60, 90, 120, and 180 minutes followed by oil quenching. Heat treatment in general improved the wear resistance over that in the as-cast (as-received) state. The extent of maximum improvement differed with temperature and in the decreasing order occurred at (1) 180 minutes, 800 °C, OQ; (2) 30 minutes, 950 °C, OQ; (3) 90 minutes, 900 °C, OQ; and (4) 180 minutes, 850 °C, OQ. From the point of view of commercial application, the heat treatment at (2) is most favored. Microstructural changes occurring during heat treating comprised (1) changes in matrix microstructure; (2) a reduction in volume fraction of massive carbides due to its part graphitization/destabilization; and (3) changes in graphite morphology, size, and distribution. Amongst the aforesaid changes, graphitization has emerged as the key parameter in improving wear resistance. Graphite morphology in a near-nodular form of optimum size and distribution was found to be most effective. Upon increasing the heat-treating temperature, the tendency of nodules to develop spikes increased. Similarly, interlinking of graphite flakes was also observed. These features and the possible formation of free ferrite adversely affected wear resistance. The role of other beneficial changes in the microstructure, e.g., globularization of carbides, possible retention of austenite, and formation of optimum volume fraction of martensite, have been duly considered while optimizing microstructure(s). The key feature of the present study is that, despite its fundamental significance, it has a well-focused application potential.     

Fracture toughness of powder forged Cr-Mn alloy steels

Metallurgical and Materials Transactions A - The fracture characteristics of a powder forged Cr-Mn alloy steel have been evaluated by plane strain fracture toughness tests. The toughness values...     

The Effect of shock hardening on the impact resistance of low-carbon steel

Mild steel plates were deformed by explosive loading prior to the testing of notched specimens in slow and impact bending and of plain specimens in uniaxial tension. The shock loading produced a significant increase (up to 25 ksi) in tensile yield strength σ _Y . The increase resulted from both a high dislocation density and a high density of mechanical twins that were introduced by shock loading. At the same time, the slow-bend and impact transition temperatures were unaffected by the shock hardening. Elastic-plastic analysis showed that the shock loading results in an increase inσ _? ^* , the critical tensile stress required to initiate cleavage below the notch root. It is this increase inσ _? ^* that is responsible for preventing a rise in the brittleness transition temperature in shock-hardened plates, despite the increase inσ _Y . The increase inσ _? ^* was attributed to the increase in twin density, hence an increase in the number of twin boundaries. Although grain boundaries have a primary role in preventing the initiation or propagation of microcracks, extensive testing and analysis demonstrated that the twin boundaries also serve as barriers in this regard.     

Fracture resistance of fiber-reinforced ceramic matrix composites

The influence of the properties of the fibers, the matrix and the interface on the mechanical properties of fiber reinforced ceramics is analyzed by a simplified method previously developed by the authors for cohesive materials. The method parts from the assumption that crack displacements are known a priori and furnishes, in a simple and easy way, the fracture resistance curves versus crack length. The numerical results from the model are compared with experimental data from the literature. Finally, the model is used to assess the influence of fiber strength, interface slipping shear stress, fiber radius and fiber defect distribution on the fracture resistance and ductility of fiber-reinforced ceramic composites.     

Corrosion and wear resistance of chrome white irons—A correlation to their composition and microstructure

The corrosion and wear resistances of a series of cast chromium white irons (CWIs) were evaluated using electrochemical and low stress sliding abrasion tests. The results show clearly that corrosion resistance of these materials is largely dependent on the quantity of chromium in the matrix, while wear resistance is mainly controlled by the volume fraction of chromium carbides. Based on theoretical analysis, a wear/corrosion performance map is established to identify alloy compositions that may be suited for erosion/corrosion conditions.     

Fracture of a low-carbon steel under mode I,mode II,and mixed-mode loading conditions

The effect of the shear component of static loading on the evolution of the plastic deformation zones and the mechanical and acoustic properties (acoustic emission parameters, ultrasonic wave velocity and attenuation coefficient) of a low-carbon grade 20 steel is studied. It is found that an increase in the shear loading component leads to a change in the shape of a plastic deformation zone, the appearance of an additional system of microcracks, an increase in the total fracture energy, a decrease in the slopes of the cumulative distributions of the acoustic signal amplitude and the microcrack length, and a significant increase in the ultrasound attenuation coefficient.     

Fracture resistance of endodontically treated roots after restoration

This study evaluated the shear strength resistance of endodontically treated roots that were restored by two different techniques. Twenty-seven recently extracted single-rooted teeth with similar anatomic characteristics were sectioned to obtain the same length for all specimens. Group I (GI) consisted of 14 roots restored with cast post-core (nickel-chromium alloy) and cemented using zinc phosphate cement; group II (GII) consisted of 13 roots restored with steel prefabricated posts (FKG) cemented with zinc phosphate cement and rotated with caution for anchorage. The crown portion of this group was made using a hybrid composite resin (Prisma APH). Results showed that on all specimens of GI the fracture occurred in the cervical root structure while in GII the composite resin fractured in all specimens. Statistical analysis showed a significantly higher resistance to fracture for GI than GII. The specimens were sectioned longitudinally for stereoscopic microscope analysis (63x) and did not show fracture lines in the dentin anchorage post area for any of the specimens.