THE EFFECT OF NITROGEN AND MICROALLOYING ADDITIONS OF VANADIUM AND TITANIUM ON MECHANICAL PROPERTIES OF 40Cr7 STEEL

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1 Renata Staśko*, Henryk Adrian** THE EFFECT OF NITROGEN AND MICROALLOYING ADDITIONS OF VANADIUM AND TITANIUM ON MECHANICAL PROPERTIES OF 40Cr7 STEEL ABSTRACT The effect of nitrogen and microalloying additions of V and V+Ti on the mechanical properties of heat treated 40Cr7 steel was investigated using four laboratory steels. Two variants of heat treatment were applied: normalizing and quenching with tempering at 600 C. Mechanical properties - using tensile and notch toughness tests as well as hardness measurements - of heat treated steels were investigated. After normalizing higher values of tensile strength in non microalloyed steels were observed whilst after quenching and tempering V and V+Ti alloyed steels showed higher values of Rm. KEYWORDS: microalloying elements, high strengh low alloyed steel, mechanical properties 1. INTRODUCTION Mechanical properties of heat treated constructional steels depend on their chemical composition as well as the microstructure obtained after heat treatment. Microstructure formed during decomposition of austenite depends on the hardenability of steel and on cooling rate. The best combination of tensile and plastic properties of steel are obtained for temper martensite microstructure [1]. Higher hardenability enables martesitic transformation of austenite during slow cooling. Low cooling rate decreases the internal stresses formed during heat treatment. The hardenability of steel depends on its chemical composition. In quenched and tempered steels the common alloying elements - chromium or manganese - are used for hardenability increase [1]. Addition of chromium showing higher chemical affinity for carbon compare to manganese enables to increase the hardness of diffusional products of austenite decomposition. Addition of microalloying elements of V and Ti in chromium alloyed steel with higher nitrogen content result in decreasing the austenite grain size and finally in decrease of grain size of austenite decomposition products. The aim of present work was to investigate the effect of nitrogen and microalloying additions of V and V+Ti on the mechanical properties of heat treated 40Cr7 steel. mgr Renata Staśko, Faculty of Mathematics, Physics and Technics, Academy of Education, Kraków, ul. Pochorążych 2, rstasko@ap.krakow.pl ** dr hab. inż. Henryk Adrian, prof. AGH, Faculty of Metals Engineering and Industrial Computer Science, University of Science and Technology, AGH, Kraków, Al. Mickiewicza 30, adrian@uci.agh.edu.pl Financial supports of KBN - grant nr 4 T08B and AP - grant BW 242/M/2006 are acknowledged

2 2. MATERIALS AND METHODS The investigated steels were air melted in a 20 kg laboratory induction furnace and poured at temperatures C. The ingots were surface dressed and hot rolled to 40 mm square bars. The chemical composition of these steels are given in table 1. Steels N1 and N2 have different nitrogen contents with trace amounts of microalloying elements whilst steel V-N2 contains % V and steel V-Ti-N2 contains 0.084% V, 0.02% Ti and both steels have increased content of N. Table LChemical composition o f steels, in weighty % Stal C Mn Si N Ti V Nb Cr A1 P S N1 0,39 0,91 0,28 0,0038 0,012 <0,004 <0,006 1,92 0,006 0,008 0,006 N2 0,38 0,97 0,29 0,032 0,003 <0,004 <0,004 1,84 0,009 0,009 0,006 V-N2 0,36 0,93 0,27 0,0412 0,003 0,078 <0,004 1,88 0,013 0,009 0,006 V-Ti-N2 0,39 0,96 0,27 0,0370 0,020 0,084 <0,006 1,95 0,02 0,010 0,009 From each steel bar the piece of length of 120 mm was taken for heat treatment. All bars were normalised at 850 C for 1 h. After normalizing part of bars were austenitisied at 870 C for 40 min., quenched in water and then tempered at 600 C for 1 h with following cooling in water. This heat treatment enabled to compare mechanical properties of investigated steels after two variants of heat treatment: N - normalizing, Q+T - quenching and tempering. After heat treatment the specimens for tensile and impact testing were prepared. The tensile specimens had diameter, d0=5 mm and length, 10=25 mm. For impact test the standard Charpy V specimens were used. Tensile test was carried out using MTS 810 machine coupled with computer for data storage. From tensile tests standard properties were calculated: yield stress, Re, tensile strength, Rm, total elongation, A5 and reduction of area, Z. For impact testing two specimens were used for one point and mean values for each point were calculated. Part of impact specimens were used for metallographic investigations. In order to reveal the microstructure prepared surfaces were etched in nital (3 % HNO3 solution in alcohol). The microstructure was observed by Leica light microscope. Also Rockwell hardness was measured using three measurements for each specimen and mean values were calculated. 3. RESULTS 3.1. MICROSTRUCTURE Examples of the microstructures of investigated steels are presented in figs. 1 and 2. After normalizing the microstructure of non microalloyed steels N1 and N2 consists of martensite and bainite whilst in microalloyed steels V-N2 and V-Ti-N2 the microstructural components are pearlite and ferrite (fig. 1). The microstructure of quenched and tempered steels consists of tempered martensite (fig. 2).

3 c) d) Fig 1. Typical microstructures of normalised steels a) NI, b) N2, c) V-N2, d) V-Ti-N2 a) b)

4 Fig 2. Examples of microstructures of quenched and tempered steels a) NI, b) N2, c) V-N2, d) V-Ti-N MECHANICAL PROPERTIES The mechanical properties of heat treated steels are presented in table 2. Tabela 2. Mechanical properties o f steels (N-normQ+T- quenching and te Steel Heat Re Rm Ar a5 Z HRC KV treatment [MPa] [MPa] [%] [%] [%] J/cm2 N1 N N , V-N , V-Ti-N , N1 Q+T , N , V-N , V-Ti-N , Tensile test curves are presented in figs. 3 and 4. After normalizing a significant differences between tensile curves for non alloyed and microalloyed steels are observed (fig. 3). After normalizing of microalloyed steels, V-N2 and V-Ti-N2, yield points occurred while in non alloyed steels NI N2 there is no yielding. Generally after normalizing non alloyed steels show higher strength as well as hardness, whilst their plastic and impact properties are lower compare to microalloyed steels. It is worth to note the very high impact strength of steel V-N2 (195 J/cm2), whilst in case of steel V-Ti-N2 this parameter is equal 55 J/cm2. Impact strength for non alloyed steels are in the range of J/cm2. The tensile strength of non alloyed steels are 1280 and 1209 MPa whilst in case of microalloyed steels this parameter is in the range of MPa. After quenching and tempering all steels had not yield points. Strength properties of microalloyed steels (V-N2 and V-Ti-N2) are higher then non microalloyed steels (N1 and N2). Tensile strength for non microalloyed steels was in the range of MPa whilst for microalloyed steels - in the range of MPa. It is worthy to emphasize, that in not microalloyed steels the increase of nitrogen content resulted in higher values of Rmand Re. The highest value of impact strength showed steel N1 whilst the lowest - steel V-Ti-N2.

5 Fig. 3. Tensile test curves for normalised steels Fig. 4. Tensile test curves for quenched and tempered steels 4. DISCUSSION OF RESULTS After normalizing the nitrogen decreases strength properties and increases plastic properties as well as impact strength. These variations of mechanical properties are related to the effect of nitrogen on hardenability of investigated steel. It was proved that nitrogen decreases hardenability of investigated steel [2], At conventional austenitising temperature further decrease of hardenability of microalloyed steel occur as a result of higher content of carbonitride M(C,N) precipitations. The precipitates promote the nucleation for diffusional products of austenite decomposition. In case of normalised microalloyed steels the microstructure consists pearlite and ferrite whilst in non microalloyed steels - martensite and bainite. For steel V-N2 very high impact strength was observed, about four times higher then for V-Ti-N2. It can be explained by lower contents of small V(C,N) carbonitride precipitated in ferrite compare to V-Ti-N2 steel. This is because addition of Ti showing higher chemical

6 affinity both to N and C increases the dissolved in austenite vanadium content and during cooling higher amount of V(C,N) carbonitride can precipitate in ferrite. As a result higher hardnes of V-N2 steel in comparison to V-Ti-N2 is observed. After quenching and tempering higher tensile strength show microalloyed steels V-N2 and V-Ti-N2 compare to non alloyed steels. This is because of the effect of dissolved vanadium content after quenching of microalloyed steels [3], During tempering dissolved in martensite vanadium can form carbonitride precipitations, giving the precipitation hardening effect. It is worthy to note, that at similar hardness and plastic properties of N2 and V-N2 steels the second one show 92 MPa higher value of Re showing beneficial effect of vanadium on the mechanical properties of steel with increased nitrogen content. Simultaneous addition of V and Ti results in further increase of mechanical properties of quenched and tempered 40Cr7 steel. 5. CONCLUSIONS a) Due to higher hardenability, non alloyed steels have higher tensile strength in normalising condition then steels microalloyed with V and V+Ti b) After normalising V-N2 steel showed four times higher impact strength than V-Ti-N2 c) Quenching and tempering result in higher tensile strength of microalloyed steels in comparison with non alloyed steels d) The nitrogen increases the tensile strength Rmof quenched and tempered 40Cr7 steel REFERENCES 1. T. Malkiewicz: Metaloznawstwo stopów żelaza, Państwowe Wydawnictwo Naukowe, Warszawa-Kraków, H. Adrian, R. Staśko: HUTNIK-Wiadomości Hutnicze, 2004, nr 3, s.l H.Adrian: Model termodynamiczny wydzielania węglikoazotków w stalach niskostopowych o podwyższonej wytrzymałości z zastosowaniem do badań; Rozprawy. Monografie Nr 18, Wydawnictwa AGH, Kraków, 1995