THE EFFECT OF THE METHOD AND PARAMETERS OF THEHEAT TREATMENT ON ABRASIVE WEAR RESISTANCE OF 38GSASTEEL

Abstract

This paper presents the structure and the results of abrasive wear resistance testing for 38GSA steel in an asdelivered condition (after heat refining) and after volume hardening. Based on the tests conducted by both light and scanning microscopy methods, it was demonstrated that, due to the performed technological operations, this steel differed significantly in terms of structure compared to the as-delivered condition, which affected its performance characteristics. In an as-delivered condition, 38GSA (38MnSi4) steel is characterised by a finegrained ferrite-pearlite structure with martensite areas arranged in bands, which significantly differs from the structure typical of the state of equilibrium. After volume hardening, the steel in question is characterised by a homogeneous fine-stripped martensite structure with clearly visible former austenite grain boundaries. The obtained results of structural testing on 38GSA steel were related to the actual abrasive wear resistance indices obtained by the “rotating bowl” method using various abrasive soil mass types. Tests conducted in the following soils, i.e. light (loamy sand), medium (light loam) and heavy (common loam), including hardness measurements, showed a close relationship between the results obtained for abrasive wear resistance and the phase structure resulting from the heat treatment state of the tested material. The obtained results of the tests on 38GSA steel were compared to those for low-alloyed martensitic abrasive wear resistant steels Hardox 500 and Brinar 500. Słowa kluczowe: obróbka cieplna, intensywność zużycia, masa glebowa, stal 38GSA. Streszczenie W pracy przedstawiono budowę strukturalną oraz wyniki badań odporności na zużywanie ścierne stali 38GSA w stanie dostarczenia (po wyżarzaniu normalizującym) oraz po hartowaniu objętościowym. Na podstawie przeprowadzonych badań metodami mikroskopii świetlnej i skaningowej wykazano, że w wyniku wykonanych operacji technologicznych stal ta cechuje się znaczną różnicą w budowie strukturalnej, w stosunku do stanu dostarczenia, rzutującą na jej charakterystyki użytkowe. W stanie dostarczenia stal 38GSA (38MnSi4) charakteryzuje się drobnoziarnistą strukturą ferrytyczno-perlityczną z pasmowo ułożonymi obszarami martenzytu, znacząco odbiegającą od struktury charakterystycznej dla stanu równowagi. Po hartowaniu objętościowym omawiana stal cechuje się jednorodną strukturą drobnolistwowego martenzytu z wyraźnie widocznymi granicami ziaren byłego austenitu. Uzyskane wyniki badań strukturalnych stali 38GSA odniesiono do rzeczywistych wskaźników odporności na zużycie ścierne, uzyskanych metodą „wirującej misy”, wykorzystując różne glebowe masy ścierne. Zrealizowane badania w glebie lekkiej (piasek gliniasty), średniej (glina lekka) oraz glebie ciężkiej (glina zwykła), a także przeprowadzone pomiary twardości wykazały ścisłą zależność uzyskanych wskaźników odporności na zużywanie ścierne od budowy fazowej wywołanej stanem obróbki cieplnej badanego materiału. Uzyskane wyniki badań stali 38GSA odniesiono porównawczo do niskostopowych, martenzytycznych stali odpornych na zużywanie ścierne Hardox 500 i Brinar 500. * ORCID: 0000-0002-9587-8355. Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland, e-mail: [email protected]. ** ORCID: 0000-0003-2953-7402. University of Warmia and Mazury in Olsztyn, Michała Oczapowskiego 2, 10-719 Olsztyn, Poland, e-mail: [email protected]. INTRODUCTION An analysis of data concerning the criteria for the selection of materials for heavily-loaded structural elements of selected machines used, e.g., in mining, agriculture, or transport, indicates that the main factors limiting their service life are intense abrasive wear processes and dynamic loads. From the materials science perspective, the above-mentioned limitations can be formulated, as cited in the study [L. 1], in accordance DOI: 10.5604/01.3001.0013.5435 62 ISSN 0208-7774 T R I B O L O G I A 3/2019 with the following criteria: abrasive wear resistance, random load resistance, microstructure homogeneity and properties of the entire component’s cross-section, and the possibility for joining using welding techniques. Other scientific papers [L. 2–4] indicate hardness, chemical composition, microstructure, and the operating environment properties as the main factors determining the course of structural material wear. As regards soil mass, its acidity, grain size, compactness, moisture content, and the size and shape of actively interacting grains are defined. These studies also indicate that abrasive mass grain size is the basic factor that determines the manner in which it interacts with the material, which is additionally intensified when the operating speed and unit load increase and when dynamic loads are present. The above-mentioned requirements are fulfilled by low-alloyed martensitic steels due to their favourable performance properties. They are manufactured based on precisely selected chemical composition (determined by the sheet thickness), low harmful additive content, and the homogeneous microstructure over the entire steel semi-finished product’s cross-section, which is obtained due to specialised thermoplastic treatment. The leading manufacturers of these materials include the following steel plants [L. 5, 6]: SSAB-Oxelösund (Hardox), ThyssenKrupp Steel Europe AG (TBL and XAR®), Grobblech GmbH (Durostat and Brinar), Industeel (Fora and Creusabro), Tata Steel Group (Abrazo), Titus Steel (Endura), Sumimoto Metal (Sumihard) and many others. In Poland, the Hut-Trans Katowice steel plant offers steels under the trade name of HTK. It is also worth including 38GSA steel (38MnSi4 according to standards BN-85/0642-48 and EN 10083-2) in the above list, which was developed in 1985 for the purposes of domestic agricultural industry. This steel, despite the fact that it is no longer manufactured, is still commercially available and used primarily for ploughshares and other parts operating in an abrasive steel mass. 38GSA steel has already been the subject of many studies, e.g., [L. 3, 4, 7, 8]; however, in each of the cases under consideration, the results of tests on this steel referred only to the normalised state which, in many instances, prevents a reliable assessment of this material in terms of resistance to abrasive wear processes, particularly in relation to other modern martensitic steels. In view of the above issue, this paper is a follow-up to research on 38GSA (38MnSi4) steel which takes into account the delivery condition for this steel. RESEARCH MATERIAL AND METHODOLOGY The study used sheets of 38GSA steel manufactured using the hot rolling technology and subjected to heat refining under metallurgical conditions. Specimens for the structural tests and abrasive wear resistance tests were taken in the form of rectangles with dimensions of 30x25x10 mm using methods ensuring their structure stability. Some of the specimens were then subjected, under laboratory conditions, to heat treatment procedures by means of water volume hardening and lowtemperature tempering. The heat treatment procedures were performed in a gas-tight chamber furnace FCF 12SHM/R by Czylok under inert gas (99.95% argon). The heat treatment parameters were selected based on the requirements of industry standard BN-85/0642-48 and the actual chemical composition of the analysed steel, which at the same time enables obtaining a homogeneous martensitic structure over the entire cross-section of the specimens. The applied parameters were as follows: austenitising temperature – TA = 880°C, austenitising time – tA = 20 minutes, tempering temperature – TO = 200°C, tempering time – tO = 180 minutes, cooling medium (H2O) temperature – TW = 30°C. After the tempering, cooling was carried out in the open air. Five specimens were prepared for each of the test variants. Hardox 500 and Brinar 500 steels (which are similar to 38GSA steel in terms of the chemical composition and properties) were selected as the reference material for the analysed steel. To cut out laboratory specimens, the high-energy abrasive water jet method and wire EDM cutting were applied. Tests for resistance to abrasive wear were conducted by the “rotating bowl” method using a MZWM−1 device. Considering that the general design and the schematic diagram of the device have already been discussed extensively, e.g., in studies [L. 3, 4, 7–10], this paper avoided their re-presentation. Table 1 presents only the basic soil parameters in which the research experiments were conducted. Table 1. Characteristics of the abrasive soil mass (Polish Soil Science Society (PTG) classification, 2008) Tabela 1. Charakterystyka glebowej masy ściernej (klasyfikacja PTG 2008) Soil mass type Granulometric group Fraction content [%] Moisture content by weight [%] Sand 2.00–0.05 mm Dust 0.050–0.002 mm Silt < 0.002 mm LIGHT Loamy sand 82.7 8.4 8.9 10–12 MEDIUM Light loam 58.3 22.5 19.2 11–13 HEAVY Common loam 38.0 35.7 26.3 12–15 63 ISSN 0208-7774 T R I B O L O G I A 3/2019 Tests on abrasive wear resistance were carried out applying the following friction parameters: a velocity of 1.66 m/s, a friction distance of 20 000 m, and a unit pressure of 67 kPa. The mass wear was measured every 2000 m. During the testing, a slightly acidic pH of the soil (pH = 6.4–6.8), measured by the electrometric method using an EpH-117/118 meter by Alsmeer-Holland, was ensured. The moisture content of the soil was determined using the oven-drying method by measuring the weight of the solid phase dried at a temperature of 105°C, according to Formula (1):