Decarburization of hypoeutectoid carbon steels during isothermal annealing in air at temperatures Ac1 < T < Ac3

Raziskovalo se je razogljičenje svetlo brušene površine nelegiranih podevtektoidnih jekel C22, C45 in C60 pri različno dolgem žarjenju na temperaturah AC1 TG. Pri tem je potrebno upoštevati najmanjši znani difuzijski koeficient ogljika v avstenitu. Za eksperimentalne pogoje žarjenj jekel C22, C45 in C60 so bile iz omenjene enačbe razvite poenostavljene enačbe za izračun debelin razogljičenih plasti. Primerjava med izmerjenimi in teoretično izračunanimi debelinami razogljičenja je pokazala zelo veliko ujemanje rezultatov.Decarburization of hypoeutectoid carbon steels during isothermal annealing in air at temperatures Ac1 TG. The lowest known value of the diffusion coefficient of carbon needs to be used with this equation. Equations, that can be used for calculation of the theoretical depths of decarburization for experimental conditions used in this thesis, were created from Van-Ostrand-Dewey equation. The comparison between measured and calculated results shows high levels of matching

Development and characterization of iron and manganese alloys for brazing of non-alloy and low-alloy steels

V sklopu raziskave smo razvili, karakterizirali in preizkusili nove dodajne materiale na osnovi Fe-P in Mn-Fe-P za spajkanje nelegiranih in malolegiranih jekel. Ti imajo relativno nizke temperature solidus in likvidus ter ozke talilne intervale in so poceni za izdelavo. Po razmisleku o optimalni sestavi s pomočjo pridobljenih že znanih faznih diagramov iz literature in podrobne metalurške analize smo ulili 22 zlitin iz 12 različnih zlitinskih sistemov. Po preliminarnem pregledu smo izbrali 19 zlitin iz devetih različnih zlitinskih sistemov, s katerimi smo nadaljevali raziskavo. Tem smo analizirali mikrostrukturo v litem stanju, z DSC analizo določili temperature solidus in likvidus ter talilne intervale, izmerili kemijske sestave in opravili meritve mikrotrdot. V sklopu preizkusov omočljivosti smo zlitine sprva analizirali na ogljikovem jeklu C22 na 1080 °C v atmosferah argona ter kombinacije vodika in dušika. Z dobljenimi rezultati smo naredili končen izbor najobetavnejših sedmih zlitin. Sledile so meritve omočljivosti pri 1000 °C, 1040 °C v Ar na jeklih C22 in 15CrNiS6 ter na 1080 °C na jeklu 15CrNiS6 v obeh atmosferah. Vzorcem po preizkusu omočljivosti smo analizirali mikrostrukture in izmerili mikrotrdote faz. Sledili sta spajkanje obeh jekel v atmosferi vodika in dušika ter difuzijsko spajkanje izhodiščnih spojev v atmosferi argona. To je potekalo 2 uri in 8 ur. Spoje smo mikrostrukturno analizirali s svetlobnim in z vrstičnim elektronskim mikroskopom ter EDS detektorjem, pregledali in opisali interakcije med dodajnimi in osnovnima materialoma ter premerili mikrotrdote po presekih spojev. Spoje smo na koncu strižno preizkusili. Izdelali smo tudi difuzijsko spajkane spoje pod tlačno obremenitvijo 20 MPa v času 10 minut, ogrevane s plamenom C2H2/O2 ob uporabi komercialnega talila. Dokazali smo, da so novi dodajni materiali na osnovi Fe-P in Mn-Fe-P primerni za spajkanje ogljikovih in malolegiranih jekel, saj tvorijo kakovosten spoj brez prisotnosti intermetalnih faz v osnovnem materialu zaradi difuzije fosforja, bora, ogljika ali silicija. Strižne trdnosti so v zgornjih dveh tretjinah običajnih vrednosti v tehniki. Difuzijsko spajkanje je spoje na osnovi Fe-P dodatno izboljšalo in jih naredilo duktilne. Spoji, narejeni s spajkami na osnovi Mn-Fe-P, po difuzijskem spajkanju niso pokazali izboljšanja. V splošnem smo dokazali, da so izdelane in preizkušene spajke primerne za uporabo v tehniki.New Fe-P-based and Mn-Fe-P-based filler metals for brazing of non-alloy and low-alloys steels were developed, characterized and tested in this research. They have relatively low solidus and liquidus temperatures as well as narrow melting ranges and are cheap to produce. After a short deliberation about optimal chemical compositions with the help of already existent phase diagrams and a metallurgical analysis, 22 new alloys from 12 different alloy system were cast. 19 alloys from 9 systems were chosen to continue the research with after a preliminary check. These had their microstructures analysed, solidus and liquidus temperatures as well as melting ranges determined by DSC analysis, chemical compositions measured and microhardnes measurements performed. In the scope of wettability test, these alloys were first tested on C22 carbon steel at 1080 °C in both argon as well as nitrogen + hydrogen atmospheres. Data given by these tests was used to make a final selection of 7 of the best performing filler metals. Wettability tests at 1000 °C, 1040 °C in Ar on C22 and 15CrNiS6 steels as well as at 1080 °C on 15CrNiS6 in both atmospheres followed. These samples had their microstructures analysed and microhardnesses measured. Brazing with new alloys in nitrogen + hydrogen atmosphere was performed next. Diffusion brazing of joints created by aforementioned process for 2 h and 8 h in Ar was also performed. Brazed joints were microstructurally analysed with optical and electron microscopes as well as an EDS detector. Interactions between filler metals and base materials were examined and microhardnesses in the joints measured. In the end, joints had their shear strengths measured. Created brazed joints were also preliminarily used for diffusion brazing under 20 MPa of compressive load. These were torch brazed for 10 minutes. The tests have proven, that new Fe-P-based and Mn-Fe-P-based filler metals are suitable to be used for brazing of carbon and low-alloy steels, as they form a good joint without any intermetallic phases in the base metals due to the diffusion of P, B, C and Si. Shear strengths are in upper two thirds of usual values found in the literature. Diffusion brazing has improved Fe-P-based joints and made them ductile. Joints made with Mn-Fe-P-based brazes have shown no improvement after diffusion brazing. In general, it was proven that new materials are suitable to be used as brazes

Analysis of the mixing of filler and base materials in arc-welded single-bead surface welds using an EDXS method

This article deals with an analysis of mixing and determines the admixing rate of a base S355 steel plate in single-bead surface welds by measuring the chemical composition using a planescan energy dispersive X-ray spectroscopy (EDXS) on metallographic cross-sections. The results show that obtaining a larger number of EDXS measurements does not necessarily lead to obtaining a more accurate admixing rate. Due to the ever-present segregations that are generally near the base material, the disadvantage of this method is the subjective influence of the SEM operator on the estimated admixing rate. To obtain relevant results, a sufficiently wide area of well-mixed melt, including segregations, must be analyzed. This study showed that by using a sufficiently large number of appropriately selected sites with a sufficiently large surface area, it is possible to estimate the admixing rate from the chemical composition with an accuracy of ≥96% for the geometrically determined admixing rate D = 30%. From several equations, the best result showed an equation which is the arithmetic mean of the two different arithmetic means and in which the artificial influencing factor of the segregations of the base material is taken into account. With this equation, the same value of admixing rate, D = 30%, was obtained using the comparative geometric method

Determining the degree of admixing rate of the base material and the melting efficiency in single-bead surface welds using different methods, including new approaches

The precise determination of the admixing rate of the base material for certain welding parameters is very important because of the possible negative consequences. As such, it is the basis for corrections in welding technology. In the article, experimental and theoretical determinations of the admixing rate in single-bead surface welds that were arc welded onto S355 steel with different alloyed-steel-coated electrodes are discussed. The admixing rate was experimentally estimated from the ratio of the surface areas of metallographic cross-sections, from the ratios of the height and from chemical analyses of different regions of the surface weld, while it was theoretically estimated from the characteristics of the welding process and material constants. One of the key characteristics of the welding process is the melting efficiency, which can be estimated by means of different equations and from knowledge of the heat balance of the welding process. Both the average melting efficiency of the surface welding on the medium-thick S355 steel plate and the average admixing rate of the S355 steel into the surface welds have the same value, i.e., approximately 30%. New equations for estimating the melting efficiency of the arc welding with a coated electrode were developed on the basis of the results

Decarburization of the Carbon Steel C45 During Annealing in Air

In production it is necessary to achieve conditions that lead to the minimum decarburization of a steel product’s surfaces. In this study, the hypo-eutectoid carbon steel C45 was annealed in air in the temperature range Ta = 600–1100 °C. The annealing times were between ta = ½ h and ta = 2 h. Different decarburizations occurred in different microstructures: ferrite–pearlite (Ta = 600 °C and 700 °C, Ta 912 °C, visible decarburization and overheating of steel). The edges were more prone to decarburization and to overheating. Stress relieving, normalizing and annealing before quenching of the steel C45 can be carried out in air

Effect of the shape of styrene-acrylonitrile water-filter housings on the destructive pressure, crack-initiation, propagation conditions and fracture toughness of styrene-acrylonitrile

A destructive pressure test of styrene–acrylonitrile (SAN) water-filter housings showed the influence of the shape and specific details of the housings on their critical areas and their destructive pressure. The destructive pressure varies by as much as 37 bar due to different dominant stresses in the individual types of housings. In critical areas of the housings, geometrical stress concentrators generally exist. For this reason, the stress caused by the internal pressure is locally 2.75–3.4 times greater than that expected based on the water pressure, which means that cracks are initiated in these places. However, the bottom of the housings can be in a form such that the maximum stress and the crack originates in its central part without the influence of local stress concentrators. The tensile strength of the SAN is theoretically estimated at 73 N/mm$^2$, which is comparable with the literature data. The fracture toughness of the SAN is typically low, theoretically estimated in the range 1.45–3.55 MPa·m$^{1/2}$, and strongly depends on the degree of the wall’s stress-increasing rate or the crack-propagation rate. Therefore, at various crack-propagation rates, the critical crack depths are also different, in the range 100–600 µm. Due to this, the critical thickness for brittle fracture in the SAN is also differentit is ten times greater than the critical crack length. The characteristic of a sub-critical crack, i.e., the mirror zone, is its macroscopically smooth surface, which is microscopically very finely roughened. In the case of a sufficiently slowly growing sub-critical crack, the surface of the mirror zone contains characteristic parabolic markings. The over-critical, sufficiently rapidly growing cracks generally grow mainly in the plane-strain state and only the final thin layer of the remaining wall thickness breaks in the plane-stress state. The over-critical, sufficiently slowly growing cracks grow in the plane-stress state with a strong shear plastic tearing

A Preliminary Study of New Experimental Low-Cost Fe–P-Based and Mn–Fe–P-Based Brazing Filler Metals for Brazing of Non-Alloy and Low-Alloy Steels

Seventeen new experimental filler metals from eight different alloy systems based on Fe–P–X and Mn–Fe–P–X (X = B, C, Si in various combinations) were created and experimented with. DSC analyses were performed to determine the solidus and liquidus temperatures and the melting ranges. Hardness measurements of the alloys were performed in the as-cast state. The alloys contain primary and eutectic intermetallic compounds that make them very hard with average hardness values ranging from 590 HV10 to 876 HV10. The wettability was determined at 1000 °C, 1040 °C and 1080 °C on C22 non-alloy steel and 15CrNiS6 low-alloy steel in Ar 4.6 and 78 vol% H2-22 vol% N2 atmospheres. The results show good wettability at T = 1080 °C in both atmospheres, as the contact angles were mostly ≤30°. Thirteen alloys exhibit very good wettability with average contact angles of ≤15.5°. Nine alloys exhibit excellent wettability with their average contact angles being ≤10°. Wettability improves at higher temperatures. The liquid alloys are reactive to solid steels and form a diffusion joint. Diffusion of P, B, C, and Si from the filler metal into the base material dealloys the composition of the melt near the joint interface. For the same reason, a continuous layer of solid solution forms on the joint interface. When brazing with filler metals rich in carbon, strong carburisation of steels can be observed near the joint

Determining the Degree of Admixing Rate of the Base Material and the Melting Efficiency in Single-Bead Surface Welds Using Different Methods, Including New Approaches

The precise determination of the admixing rate of the base material for certain welding parameters is very important because of the possible negative consequences. As such, it is the basis for corrections in welding technology. In the article, experimental and theoretical determinations of the admixing rate in single-bead surface welds that were arc welded onto S355 steel with different alloyed-steel-coated electrodes are discussed. The admixing rate was experimentally estimated from the ratio of the surface areas of metallographic cross-sections, from the ratios of the height and from chemical analyses of different regions of the surface weld, while it was theoretically estimated from the characteristics of the welding process and material constants. One of the key characteristics of the welding process is the melting efficiency, which can be estimated by means of different equations and from knowledge of the heat balance of the welding process. Both the average melting efficiency of the surface welding on the medium-thick S355 steel plate and the average admixing rate of the S355 steel into the surface welds have the same value, i.e., approximately 30%. New equations for estimating the melting efficiency of the arc welding with a coated electrode were developed on the basis of the results