In this paper the development of a banded microstructure in hot-rolled 34CrNiMo6 steel which consisted of bainitic and martensitic bands is explained. The chemical compositions of the bands were measured with energy dispersive x-ray spectroscopy (EDS), which showed that the martensitic bands contained more alloying elements (Mn, Cr, Mo, Si) than bainitic bands. By using Oberhoffer reagent, the segregations of phosphorus were also revealed. These phosphorus segregations coincided with the positive segregations of the alloying elements. The continuous cooling transformation (CCT) diagrams of steel were calculated. They confirmed the formation of martensite in positive segregations and the formation of bainite in negative segregations

A. Nagode

A. Resnik

B. Karpe

B. Kosec

B. Zorc

G. Kosec

I. Budak

M. Bizjak

English

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329METALURGIJA 55 (2016) 3, 329-332A. NAGODE, A. RESNIK, M. BIZJAK, G. KOSEC, B. KARPE, I. BUDAK, B. KOSEC, B. ZORCDEVELOPMENT OF BANDED MICROSTRUCTURE IN 34CrNiMo6 STEELReceived – Prispjelo: 2015-06-11Accepted – Prihvaćeno: 2016-01-10Original Scientific Paper – Izvorni znanstveni radA. Nagode, M. Bizjak, B. Kosec, B. Karpe, B. Zorc, Faculty of Natural Sciences and Engineering, University of Ljubljana, Ljubljana, Slovenia; A. Resnik, Štore Steel d.o.o., Štore, Slovenia; G. Kosec, Acroni do.o.o, Jesenice, Slovenija, I. Budak, Faculty of technical Sicences, University of Novi Sad, Novi Sad, SerbiaIn this paper the development of a banded microstructure in hot-rolled 34CrNiMo6 steel which consisted of bai-nitic and martensitic bands is explained. The chemical compositions of the bands were measured with energy dis-persive x-ray spectroscopy (EDS), which showed that the martensitic bands contained more alloying elements (Mn, Cr, Mo, Si) than bainitic bands. By using Oberhoffer reagent, the segregations of phosphorus were also revealed. These phosphorus segregations coincided with the positive segregations of the alloying elements. The continuous cooling transformation (CCT) diagrams of steel were calculated. They confirmed the formation of martensite in positive segregations and the formation of bainite in negative segregations. Key words: 34CrNiMo6 steel, metallography, banded microstructure, phosphorus, CCT diagramINTRODUCTIONA banded microstructure is a microstructure which consists of alternating bands of quite different micro-structural constituents aligned parallel to the rolling di-rection of steel products 1–3]. It can have a very nega-tive effect on impact properties in both the longitudinal and transverse direction. If the banded microstructure also consists of martensite bands it is detrimental for machining and subsequent cold forming operations [2, 4, 5]. The main reason for the development of a banded microstructure lies in the chemical inhomogeneity of the austenite. Namely, during solidification the content of solute elements and impurities increases in the liquid around solidifying dendrites which leads to interden-dritic segregations 2]. However, the presence of inter-dendritic segregations does not necessarily lead to the development of microstructural banding after rolling. It`s occurrence also strongly depends on the austenite crystal grain size and especially on the cooling condi-tion from the austenite region. Since the mechanical properties of steels are significantly dependable on their microstructure and defects in it, it is very important to understand the basic cause for the development of mi-crostructural banding in order to take appropriate action to minimise it`s occurrence and deleterious effect, espe-cially on the cold forming process.EXPERIMENTAL PROCEDUREThe development of a banded microstructure was investigated in heat treatable low alloy steel 34CrNi-ISSN 0543-5846METABK 55(3) 329-332 (2016)UDC – UDK 669.14.018:620.18:669.779:536.717=111Mo6 containing chromium, nickel and molybdenum. Typical applications are for structural use, where a good combination of tensile strength and toughness is neces-sary, e.g. shafts, gears, bolts etc. The chemical composi-tion of the investigated steel is listed in Table 1. It was supplied in a hot rolled bar with a diameter of 50 mm. Hot rolling started at 1 200 °C and finished at 875 °C.Table 1 Chemical composition of 34CrNiMo6 steel / wt. %ElementC Si Mn P S Cr0,35 0,29 0,59 0,012 0,017 1,54ElementMo Ni Al Cu Fe0,22 1,52 0,021 0,19 BalanceThe samples for microstructure characterisation were cut from the hot rolled steel bar in such a way that the normal to the sample surface was parallel to the axis of the bar. The samples were metallographically pre-pared and etched with ferric chloride. However, to re-veal phosphorus segregations the samples were etched with the Oberhoffer reagent [6]. Microstructure analy-ses were performed with light (LM) and scanning elec-tron microscopes (SEM), respectively. For the calculation of CCT diagrams a computer programme JMatPro 7.0 [7] was used. By using this programme the CCT diagrams for S355J2 steel with different chemical compositions were calculated. When doing the calculations a crystal grain size of 9 according to the ASTM E112-12 standard was taken into account.RESULTS AND DISCUSSIONThe microstructure analysis of 34CrNiMo6 steel etched with ferric chloride preformed with a light mi-croscope revealed a banded microstructure. It consisted of alternating bands of mostly bainite (dark) and mostly martensite (light), aligned parallel to the rolling direc-330A. NAGODE et al.: DEVELOPMENT OF BANDED MICROSTRUCTURE IN 34CRNIMO6 STEEL METALURGIJA 55 (2016) 3, 329-332tion of the bar (Figure 1a). However, the portion of mar-tensitic bands increased with the distance from the edge of the bar towards the centre. The microstructure observation (Figure 1a) showed that MnS inclusions (one marked with an arrow) were elongated and present in martensitic bands. Figure 1b shows the bainitic and martensitic bands at higher mag-nification. The microstructure of the same sample etched with the Oberhoffer reagent is shown in Figure 2a. This reagent revealed that the phosphorus segrega-tions (bright areas) were very intensive. From Karl [8], the lower limit of phosphorus detection using the Ober-hoffer reagent is 0,003 %. It can be seen that phospho-rus was present in alternating bands, parallel to the roll-ing direction as well. Figure 2b shows the microstruc-ture of the steel bar after etching with ferric chloride (bottom part of the sample) and Oberhoffer (upper part of the sample). It is clearly visible that phosphorus seg-regations coincide with martensitic bands. A further EDS analysis was performed. The average results in the mostly bainitic and mostly martensitic bands (Figure 3) are presented in Table 2. They indi-cated that in mostly bainitic bands the content of alloy-ing elements (Si, Cr, Mn, Ni and Mo) was lower (nega-tive segregations) in comparison to the results of the nominal chemical composition of the investigated steel listed in Table 1, while in mostly martensitic bands, the content of these elements was higher (positive segrega-tions). Note that the EDS analysis is not a proper meth-od for the measurement of light elements (e.g. carbon) and elements with a concentration below 0,1 %. Since the concentration of phosphorus was below the detec-tion limit of the EDS analysis it was not taken into ac-count. However, it is well known that manganese low-ers the activity of carbon in austenite and thus Mn–rich regions tend to attract carbon from neighbouring areas. Hence, in the Mn–rich regions, an increased concentra-tion of carbon was expected. While chromium has a similar effect to that of manganese, phosphorus increas-es carbon activity and, as a result, rejects the carbon from the phosphorus-rich regions [9].Figure 1  Banded microstructure in 34CrNiMo6 steel; a) light microscopy, b) SEM-SEIFigure 2  Phosphorus segregations; a) etched with Oberhoffer reagent (bright bands), b) etched with Oberhoffer reagent (upper part) and ferric chloride (bottom part)Figure 3  SEM micrograph of banded microstructure of 34CrNiMo6 steel marked with analysed regions331A. NAGODE et al.: DEVELOPMENT OF BANDED MICROSTRUCTURE IN 34CRNIMO6 STEELMETALURGIJA 55 (2016) 3, 329-332Table 2 Results of EDS analysis / wt. %Element Si Cr Mn Ni Mo Cu FeSite 1 0,37 1,92 0,67 1,68 0,61 0,32 BalanceSite 2 0,21 1,08 0,42 1,34 0,18 0,27 BalanceRegarding the activity of carbon, manganese and phosphorus have the opposite effect. Because the con-centration of manganese and chromium in steels is much higher than that of phosphorus, manganese and chromium has a prevailing effect on the distribution of carbon in austenite. Therefore, it is assumed that re-gions that are rich in alloying elements (Mn, Cr, Mo) and phosphorus are also rich in carbon, despite the fact that phosphorus increases carbon activity. On the other hand, the austenite regions depleted in alloying ele-ments and phosphorus contain less carbon.The CCT diagram of the 34CrNiMo6 steel with the nominal chemical composition given in Table 1 is shown in Figure 4a. However, for comparison, CCT diagrams for the same steel with the chemical composi-tions measured with EDS in the positive segregations, i.e. un the mostly martensitic bands (4b) and negative segregations, i.e., in the mostly bainitic bands (4c),were also calculated. The formation of banding in 34CrNiMo6 steel can be explained with calculated CCT diagrams. Namely, at a normal cooling rate of approximately 0,4 °C/min (bolded line in Fig. 4a) in steel with a nominal composi-tion listed in Table 1 both martensite and bainite can be expected in the microstructure. However, the alloying elements (Mn, Cr, Mo, Si) have a strong effect on the hardenability since they move the CCT diagram to the right. Hence, in regions with an increased concentration of alloying elements (positive segregations) a critical cooling rate is achieved and thus only martensite can form (Figure 4b), while in regions with a lower concen-tration of alloying elements (negative segregations) mainly bainite is expected (Figure 4c).In this calculation the changes in the concentration of carbon and phosphorus were not taken into account since they could not have been analysed with EDS. An-yway, it is known that both elements significantly in-crease the hardenability of steel since they move the CCT diagram further to the right, to a longer timing, and as such enable the formation of martensite. Phos-phorus thus significantly enhances hardenability and can even cause both quench cracking and quench em-brittlement [10, 11].Since the concentration of alloying elements as well as phosphorus and carbon increases towards the centre of the ingots, the proportion of martensitic bands was higher towards the centre of the hot-rolled bar because the concentration of these elements in austenite regions was high enough for a critical cooling rate to be achieved. Also, the martensite start temperature (Ms) was slightly lowered with an increased concentration of alloying elements.Figure 4  Calculated CCT diagrams for 34CrNiMo6 steel with a) the nominal chemical composition, b) with the composition in mostly martensitic bands (positive segregations) and c) with the composition in mostly bainitic bands (negative segregations) CONCLUSIONSA banded microstructure can be detrimental for ma-chining and subsequent cold forming operations of steel. Thus, it is essential to understand the primary reasons for its development. In this paper the development of a band-ed microstructure consisting of mostly bainitic and most-ly martensitic bands in hot-rolled 34CrNiMo6 steel was explained. The portion of mostly martensitic bands in-creased when moving from the edge of the bar towards the centre. The EDS analyses showed a higher concentra-tion of alloying elements (Mn, Cr, Mo and Si) in the mostly martensitic bands in comparison to the mostly 332A. NAGODE et al.: DEVELOPMENT OF BANDED MICROSTRUCTURE IN 34CRNIMO6 STEEL METALURGIJA 55 (2016) 3, 329-332bainitic bands. However, the EDS analyses did not show the presence of phosphorus because the quantity was be-low the detection limit of our EDS detector. Etching with the Oberhoffer reagent revealed bands of phosphorus segregations that coincided with positive segregations of alloying elements. The alloying elements (Mn, Cr, Mo, Si) have a strong effect on hardenability since they move the CCT diagram to the right. Phosphorus and carbon were not taken  into account when calculating the CCT diagram, they could not have been analysed with EDS. However, they move the CCT diagram further to the right, to a higher timing, and as such enable the formation of martensite. Since the concentration of alloying ele-ments as well as phosphorus and carbon increases to-wards the centre of the ingots, the proportion of marten-sitic bands was higher towards the centre.AcknowledgmentThe authors are grateful to prof. Božidar Šarler from the University of Nova Gorica and dr. Robert Vernik from Štore Steel d.o.o. for technical support.REFERENCES[1] B. Zorc, J. Bernetič, A. Nagode, Effects of Welding Resi-dual Stresses and Phosphorus Segregation on Cleavage Delamination Fracture in Thick S355 J2 G3+N Steel Plate, Engineering Failure Analysis 40 (2014), 8-14. [2] G. Krauss, STEELS Processing, Structure and Performan-ce, ASM International, Ohio (2005).[3] T. F. Majka, D. K. Matlock, G. Krauss, Development of microstructural banding in low alloy steel with Mn simula-ted segregation, Metallurgical and Materials Transactions 33A (2002), 1627-1637.[4] M. Muhič, F. Kosel, A. Pukšič, D. Klobčar, A new appro-ach to monitoring thermal fatigue cracks in die casting moulds, IJMR 102 (2011) 1, 69-75.[5] G. Kosec, A. Nagode, I. Budak, A. Antić, B. Kosec, Failu-re of the pinion from the drive of a cement mill, Enginee-ring Failure Analysis 18 (2011) 1, 450-454.[6] H. Schumann, Metallographie, VEB DVG, Leipzig (1987).[7] JMatPro 7.0, Sente Software Ltd.,United Kingdom.[8] A. Karl, Investigations into the Suitability of the Oberhof-fer Etch for Identifying Surface Defects, Praktische Metal-lography 15 (1978), 469-485. [9] G. Krauss, Solidification, segregation, and banding in car-bon and alloy steels, Metallurgical and Materials Transac-tions B 34B (2003), 781-792.[10] G. Roberts, G. Krauss, R. Kennedy, Tool Steels, 5th Edi-tion, ASM International, Ohio (1998).[11] F. A. Jacobs, G. Kraus,The Effects of Phosphorus and Car-bon on the Hardenability of 41xx Type Steels, Journal of Heat Treating 2 (1981) 139-146.Note:  The responsible translator for English language is Urška Letonja Grgeta, MOAR Translation, Podgora, Slovenia

Development of banded microstructure in 34CrNiMo6 steel

Development of banded microstructure in 34CrNiMo6 steel

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