INFLUENCE OF TUNGSTEN AND CARBON ADDITION ON ELECTROCHEMICAL BEHAVIOUR OF 9 % Cr CREEP-RESISTANT STEEL Jiří RAPOUCH, Jaroslav BYSTRIANSKÝ Department of Metals and Corrosion Engineering, Institute of Chemical Technology, Prague, Technická 5, 166 28 Prague 6, Czech Republic, rapouchj@vscht.cz Abstract Martensitic 9 % Cr steel belong to modern creep-resistant materials. Structural instability at higher service temperatures can lead to coarsening of particles and precipitation of new phases. These changes cause decrease of mechanical properties at high temperatures and deterioration of resistance against steam oxidation. Tungsten and carbon substantially contribute to forming of phases in the structure, most significantly of carbides and Laves phases. In this work the influence of addition of these elements on electrochemical behaviour of 9 % Cr steel with impact on active-to-passive area was studied. Results of polarization measurement were compared with hardness measurements and microstructure observation. Keywords: 9 % Cr martensitic steel, electrochemical behaviour, tungsten, polarization curves 1. INTRODUCTION Improvement of the thermal efficiency of modern power plants could be achieved by increase of steam parameters. The increase of steam temperature above 600 C also raises the requirements on creep strength of structural material. Group of materials that fulfil these demands are heat-resistant martensitic 9-12% Cr steels. The sufficient creep resistance of these steels is achieved by two steps [1, 2]: 1. Molybdenum and tungsten alloying which strengthen the solid solution, 2. alloying with vanadium, niobium, carbon and nitrogen, which contribute to precipitation hardening. The structure of 9-12% Cr steels is composed of tempered martensite, carbides M 23 C 6 and MC (M = Fe, Cr, Mo, W), nitrides and carbonitrides V(C,N) and Nb(C,N). The dislocation density in tempered state is > 10 14 m -2 [2]. During service heating some structural changes take place: coarsening of carbides, precipitation of carbonitride phases enriched with alloying elements, formation of Laves phase (Fe,Cr) 2 (W,Mo) and possible precipitation of nitride Z phase with general formula Cr(V,Nb)N [1-3]. These changes lead to general decrease of creep strength, ductility and also the corrosion resistance of the material. This paper is focused on the influence of tungsten and carbon on the structure and electrochemical behaviour of 9-12% Cr steels. Carbon in these steels improves their heat-resistance by formation of carbides and carbonitrides of alloying elements. On the other hand high carbon content causes deterioration of technological properties (formability, weldability). The amount of carbon in modern steels ranged between 0.08 0.20 wt. % [2]. Tungsten is ferrite- and carbide-forming element with higher atomic radius than iron, therefore its addition results in solid solution strengthening. In amount of 1.6 % W significantly slows down the rate of climb of dislocations and practically suppresses the precipitation of Cr 23 C 6 at grain boundaries [1]. Moreover, tungsten lowers M s temperature and makes a fine subgrain structure [4]. If its maximal solubility in steel is exceeded, tungsten precipitates forming Laves phase (Fe,Cr) 2 W and the coarsening rate of this phase is much higher than that of Cr 23 C 6 [5]. In addition, precipitation of Laves phase can be connected with formation of creep cavities and lead to intergranular fracture [6]. Tungsten together with molybdenum dissolves into M 23 C 6 carbides and thus increases their thermal stability. However, these elements reduce roughness by
forming δ-ferrite and intermetallic compounds [7]. The combined effect of W and Mo can be described by an equivalent content of molybdenum Mo(eq.) = wt. % Mo + 0.5 wt. % W. The optimal value of Mo(eq.) should range between 1.2 and 1.5 [2]. From the point of electrochemical behaviour, tungsten significantly influences the active-to-passive transition of stainless steels. The addition of W up to 8 wt. % to ferritic FeCr29 steel leads to decrease of critical anodic current density and passivation potential [8]. Moreover, it lowers the current density (and the corrosion rate) in passive state [9]. Similarly to molybdenum, tungsten enhances the resistance to pitting corrosion, its effect is approximately half compared to Mo [10, 11]. On the other hand, negative influence of W is observed in transpassive area and its presence in steel lowers the transpassivation potential of the steel. Tungsten is 2- expected to dissolve in oxidizing environments (for example in supercritical steam) as WO 4 in strong alkalies and as WO 2+ 2 in strong acids [12]. 2. EXPERIMENTAL Four different laboratory melts (2 kg) of Fe-9Cr steels with various carbon and tungsten content were used for experiments. The chemical compositions of the steels are given in Tab. 1. One set of specimens was quenched (1050 C, 20 min, air), the second was afterwards tempered at 780 C for 10 min. Tab. 1 Chemical composition samples steel chemical composition [wt. %] C Mn Si S Cr Ni Mo W C1W1.5 0.01 0.47 0.53 0.02 9.3 0.13 0.04 1.5 C15W2.3 0.15 0.54 0.45 0.15 9.3 0.13 0.03 2.3 C53W1.9 0.53 0.52 0.43 0.02 9.9 0.14 0.04 1.9 C63W4.0 0.63 0.56 0.44 0.02 9.7 0.15 0.03 4.0 All samples were grinded, polished and etched in Nital and Villela-Bain solution mixture before study of structure. Scanning electron microscope Tescan Vega 3 equipped with EDS analyser was used for microstructure observation. Vickers hardness was measured at room temperature under the load of 294 N. A common three-electrode cell with a platinum wire as counter-electrode and a silver/silver chloride electrode (ACLE) as the referent electrode (197 mv compared to the standard hydrogen electrode) was used for electrochemical measurements. All potentials reported in this paper are related to the ACLE. Aerated H 2 SO 4 solution (0.5 mol/l) at ambient temperature was used for all measurements. Potentiodynamic measurements were carried out by the PC3 system and Gamry electrochemical software. The corrosion potential was stabilized before the polarization measurement. The scan rate was 3 mv/s. 3. RESULTS AND DISCUSSION The structure of alloys in as quenched state was composed of martensitic plates, moreover, XRD analyses of these specimens showed small amounts of retained austenite (up to 6 %). Precipitation of phases preferentially on martensitic plate s boundaries occurred during tempering. Because laboratory alloys were used, their structures differ from those of standard martensitic Fe-9Cr steels. Structures of specimens C53W1.9 and C63W4.0 in as quenched and as tempered state are shown on Fig. 1.
Fig. 1 Structures of steels a) C53W1.9 as quenched, b) C63W4.0 as quenched, c) C63W4.0 as tempered The influence of steel composition and structure on hardness is plotted on Fig. 2. Quenched and tempered steels alike show increasing tendency of hardness with carbon content. In as quenched state C increases hardness of martensitic plates. In addition, W contributes to formation of fine grain structure. Carbides M 23 C 6 enriched with tungsten are formed in the structure during tempering. Moreover, increasing carbon content in the matrix leads to formation of higher amount of carbides during tempering which contributes to increase of hardness. Fig. 2 Influence of hardness on carbon content in Fe-9Cr alloy Different structural states of alloys were evaluated using electrochemical methods. Potentiodynamic curves of steels C1W1.5 and C63W4.0 are shown on Fig. 3. All measured samples in all metallurgical states hold corrosion potential around -450 mv. The active-to-passive transition is connected with development of two anodic peaks on polarization curve at potentials -250 and 0 mv. At potential 600 mv transpassive dissolution of material caused by transition of alloying elements into higher oxidation states takes place. Local peak at potential 1200 mv is expected to be connected with secondary passivation of surface. This phenomenon is induced by relative enrichment of surface with iron, which is passive in this area and causes decrease of current density [13]. The potential position of this peak is similar for samples with different content of carbon and tungsten, therefore, its occurrence is probably connected with transition of chromium from 3+ to 6+ state. The shape of this peak for as quenched and as tempered steel shows an interesting dissimilarity. This local maximum was separated into 2-3 smaller peaks in the case of tempered samples (Fig. 3, red lines). This separation could be caused by preferential dissolution of phases enriched with Cr (and other alloying elements) formed during tempering. The area of secondary passivity is not studied closer in this paper..
Fig. 3 Potentiodynamic curves of steels a) C1W1.5, b) C63W4.0 The active-to-passive area with double-peak was the main object of this work. Tempering is obvious to slightly increase current density in whole measured area, the most significant increase was observed in areas of active dissolution and secondary passivation (Fig. 3). While the first anodic peak (-250 mv) is increased only slightly, the second anodic peak (0 mv) is shifted very significantly into higher current densities. Fig. 4 compares this area for samples in quenched as well as in tempered state. The behaviour of samples in as quenched state is very similar and does not differ from that of P92 [14]. On the contrary, the current density of the first and even more significantly of the second anodic peak tend to increase with rising carbon content in the alloy, see Fig. 5. Fig. 4 Potentiodynamic curves of samples a) as quenched, b) as tempered, The occurrence of second anodic peak is assumed to be caused by several phenomena surface nickel enrichment, oxidation of absorbed hydrogen, preferential attack along phase and ferrite/martensite grains
boundaries, presence of the precipitates of Cu or Mo or dissolution of Cr-depleted areas [15, 16]. Because of high amount of carbon and chromium in used alloys Cr-depletion theory is preferred. Therefore, formation and following preferential dissolution of areas depleted with Cr surrounding carbides is considered to be the main cause of the second anodic peak on polarization curve. Some carbide particles seem to be present in the quenched matrix and their amount and size rise during tempering. This assumption is in accordance with the tendency plotted on Fig. 5. The structure was potentiostatically etched at anodic peaks potentials (-250 and 0 mv) and observed on scanning electron microscope. Fig. 5 Influence of anodic current densities at potentials -250 and 0 mv of tempered samples on C content Structure of sample C63W4.0 after potentiostatic etching is shown on Fig. 6. In as quenched state, at potential -250 mv dissolution of the matrix takes place, at 0 mv (potential of second anodic peak) significant etching of grain boundaries was observed. Carbide particles precipitate predominantly on grain boundaries and surrounded depleted zones are probably preferentially dissolved/etched at potential 0 mv. Etching of carbides at potential -250 mv in the case of tempered sample was observed. Slight enlargement of etched areas in the vicinity of these carbides was observed after etching at 0 mv. Approximate chemical composition of etched phases was detected using EDS analyzer. These phases composed of 20 24 % W, 31 37 % Cr and 40 48 % Fe (all in wt. %). Determination of accurate content of carbon is not possible using this method, but higher amount of C was detected compared to matrix. Therefore, phases M 23 C 6 (M = Cr, W, Fe) are supposed to be present after tempering and zones depleted with chromium are expected to occurs in their vicinity. Increasing second anodic current density caused by higher amount of precipitates in the structure is in accordance with hardness tendency on Fig. 2. Fig. 6 Structure of sample C63W4.0 a) quenched, etched at -250 mv, b) quenched, etched at 0 mv, c) tempered, etched at -250 mv, d) tempered, etched at 0 mv
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