ACLAND BURGHLEY SCHOOL
12
pages
English
Documents
Le téléchargement nécessite un accès à la bibliothèque YouScribe Tout savoir sur nos offres
12
pages
English
Documents
Le téléchargement nécessite un accès à la bibliothèque YouScribe Tout savoir sur nos offres
ACLAND BURGHLEY SCHOOL Learning to Succeed Together Headteacher Jo Armitage Burghley Road, London NW5 1UJ T 020 7485 8515 F 020 7284 3462 6 January 2012 Dear Candidate Thank you for applying for the post of part‐time English teacher at Acland Burghley School. The post is to cover a maternity leave and is available from January 25 to August 31 2012. The post is part‐time (0.5fte) and the days worked will be Thursday, Friday and alternate Wednesdays (we run a two‐week timetable). It may be possible to offer further supply teaching on occasion. I enclose a pack containing: 1. Copy advertisement 2. Job Description and Selection Criteria 3. Information on the English and Literacy Faculty 4. Brief facts about Acland Burghley 5. Where to find us I look forward to receiving your completed application form and a statement of no more than 2 sides of A4 by 12 noon on 16 January 2012. Please ensure that your statement demonstrates how you meet the requirements in the person specification.
Voir
- national reputation for innovation
- strong team of committed staff
- page 6 acland burghley school brief facts
- acland burghley school burghley road london nw5 1uj
- staff
Section I – Chapter 3 Metallurgy of High Chromium-Molybdenum White Iron and Steel Rolls Jean-Pierre Breyer Director Research & Development Marichal Ketin Rue Verte Voie, 39 4000 Liège Belgium Tel.: +32 4 234 72 36 rolls@mkb.beGisèle Walmag Researcher Centre for Research in Metallurgy Rue E. Solvay, 11 4000 Liège Belgium Tel.: +32 4 254 64 74 walmag@rdmetal.ulg.ac.beINTRODUCTION The Fe-Cr-C system is the basis of a number of the most widely used wear resistant materials having application in mining, mineral processing, cement works, agricultural sectors and rolling mill rolls. The iron-chromium alloys were studied as early as 1892 by F. Osmond (1) who was the first to indicate the presence of complex carbides. Other investigations have been carried out by other authors such as W. Tofaute (2,3) and K. Bungardt (4). In the frame of research works sponsored by the Climax Molybdenum, a very important development in the knowledge of the metallurgical properties of those alloys was done by F. Maratray (5,6). As concern rolling mill rolls in reviewing the history of the manufacture of rolling mill rolls, the most important innovation was probably the introduction of the double-poured indefinite chill roll in the late thirties. However, European experience may suggest that the introduction of the high chromium iron and steel rolls is likely to rank as another significant innovation in the art of rollmaking. Some Russian and American papers relate the use of high chromium iron for the manufacture of small bars and heavy section mills rolls in the early sixties (7,8). The first attempt to use that iron for hot strip mill rolls was made in Germany in 1965. Other experimental rolls were made in England a little later. It became quickly clear that despite of some failure due to the lack of reliable metallurgical information of the high chromium iron system, those rolls had an excellent resistance to both abrasion and banding. They become quickly the standard grade used in the early finishing stands of Hot Strip Mills.
Some years later, another improvement was done with the development of the high chrome steel. Carbon and chromium content were strongly decreased. Special heat treatment or alloying element additions gives a material, which suits very well to the working conditions in roughing stands of Hot Strip Mills. Today, high chrome steel rolls are used world-wide. PRODUCTION TECHNIQUE The first rolls have been produced by the double-pour method. The practice was very similar to that used for double-pour indefinite chill rolls, except that a much greater volume of flush iron must be used to reduce the chromium content of the roll core to an acceptable level. Some trials have also been done to produce those rolls single-pour and by electroslag melting of a sleeve remelted around a premachined steel arbour(7). Actually, all the high chromium iron and steel rolls are manufactured by centrifugal casting technique. The process can be vertical, horizontal or even tilted. By this technique, the small volume of the high chromium shell can solidify more rapidly than with the other casting methods thus obtaining finely dispersed carbides. It can be told that the development of centrifugal casting as a new process for rollmaking has been strongly connected to the implementation of high chromium rolls. Core is made of lamellar or spheroidal graphite (S.G.) iron. The introduction of new rolling technology such as heavy bending, shifting and crossing increases strongly the mechanical stresses in the journals and core. For that reason most of the high chromium rolls are now cast with a S.G. graphite iron core which has higher mechanical properties than lamellar iron ones. METALLURGY OF HIGH CHROMIUM IRON AND STEEL ROLLS In Fe-Cr-C alloys, chromium can substitute to iron in cementite up to a 15% content. For higher content, cementite become unstable and is replaced by an hexagonal carbide whose composition is M7C3. Those carbides called chromium carbide contain mainly chromium and iron, but other alloying element can be present. It is generally accepted that the most significant reason for the good abrasion resistance of those materials is the presence of the chromium carbides in the microstructure. Hardness of the chromium carbide is in the range 1500-1800 Vickers compared to the cementite whose hardness is in the 1000-1200 range.(12) Most of the studies of the Fe-Cr-C system are based on the diagram published by Jackson (9) and given on figure 1. Theg-M7C3area is crossed by a eutectic which is limited by two peritectic lines. Theg-dfor low carbon and high chromium content and M7C3-M3C for high carbon and low chromium content. The position of the late depends strongly of the cooling speed. High cooling speeds promote the M3C formation (10).
Fig.1 : Liquidus surface of Fe-Cr-C diagram (9) The eutectic structure depends on the amount of austenite formed at the start of solidification. When the austenite leaves only a small volume after solidification, carbide have a tendency to form along the grain boundaries as shown in figure 2.a. This is the carbide morphology of a chrome steel rolls. With carbide content of 20 to 30%, the eutectic contains lamellae radiating from the centres located in the interdendritic spaces (figure 2.b). This is the general carbide structure of a chrome iron roll as used worldwide in the early finishing stand of Hot Strip Mill. This structure changes to a lamellar one when the austenite no longer interferes with the eutectic formation. Finally, with 35 to 40% of carbides, the alloy becomes hypereutectic and large hexagonal primary carbides appear. From our knowledge, no rolls are produced with such a low toughness structure.
Fig. 2a : Typical microstructure of high Cr steel roll
Fig. 2b : Typical microstructure of high Cr iron roll A low casting temperature and a rapid solidification will decrease the size of the eutectic cells. No significant change in the morphology of the eutectic carbide occurs in during heat treatment operations, which are always performed on the roll after casting. The percentage of chromium carbides can be calculated approximately from the C and Cr content of the alloy by the following formula: % carbides = 12.33 (%C) + 0.55 (%Cr) – 15.2 The chromium content of the matrix increases regularly with the chromium/carbon ratio and can be represented by the equation : % Cr matrix = 1.95 (%Cr/%C) – 2.47 The following table gives the typical range of carbon and chromium content as well as the percentage of carbide and chromium content of the matrix measured on the high chromium grades that are used actually in hot rolling. %C %Cr % carbides Cr/C % Cr matrix Hi-Cr steel 1.0/1.5 11.0/12.0 5/15 8/10 10/14 Hi-Cr iron 2.5/3.0 16.0/18.0 25/30 5/7 6/10 The large difference in matrix composition for the two alloy families induces two distinct oxidation behaviours in the Hot Strip Mills. High chromium irons with their low Cr content in the matrix will oxidise three times faster than the high chromium steels. As concern the metallic matrix, all the decomposition product of the austenite can be produced: pearlite, bainite, martensite. A full austenitic matrix can also be completed and in many structures retained austenite is present. However, for most of the applications as rolling mill rolls of the high chromium alloys, a microstructure in which the matrix has a low residual austenite level and is free of pearlite is required. In order to get such a
structure on pieces weighting more than 7 tons and which may exceed 40 tons, different process conditions have to be put under control. Those are : ·Tempering heat treatment ·High temperature heat treatment ·Alloying elements addition Depending of their own facilities, most of the rollmakers use one or more of the above mentioned method in order to get the required structure and hardness. Those manufacturing conditions interfere between them. TEMPERING HEAT TREATMENT During a slow cooling, in the high temperature range, austenite is saturated in carbon and a precipitation of small secondary carbide may be observed (figure 3). Such a precipitation decreases the carbon and chromium content of the matrix. The pearlitic transformation is delayed and a bainito-martensitic transformation becomes possible at low temperature. Figure 4a and 4b shows the effect on the destabilisation of the austenite on the isothermal transformation diagram and on the martensitic transformation (5, 13).
Fig. 3 : Typical secondary carbides precipitation
10ïm
Fig. 4a: Schematic isothermal transformation diagrams. a)undestabilized austenite b)austenite destabilized by precipitation of secondary carbides
Fig. 4b: Effect of destabilization temperature on transformation temperatures and the Vickers hardness as air-cooled after destabilizing of a high Cr iron. It is believed that these secondary carbides, apart from their effect on the transformation of austenite to martensite contribute to wear resistance because of their high hardness and because of their dispersion hardening effect on the matrix. In the as-cast condition, the matrix contains a high percentage of austenite (30-60%) which has to be destabilised by one or several tempering heat treatment in order to get the specified hardness with a structure containing M23C6carbides in anamatrix. However there is always a stable austenite residue. If the temperature of the tempering treatment is raised in order to decompose it, hardness will be decreased. There is therefore a limit to the efficiency of those low temperature heat treatments. If a hardness of more than 80 shore C is required, it cannot be reached with a minimum of residual austenite.
Figure 5 shows for a 17% Cr – 2.8% C HSM work roll the hardness and residual austenite content evolution versus the Larson – Miller parameter P which equals: P= (273+T)(20+log t) where T is the temperature (°C) and t the soaking time (hours) of the tempering heat treatment. 750 40
700
650
600
550
500
450
35
30
25
20
15
10
5
0
400 -5 15,0 16,0 17,00 18,00 19,00 20,00 Larson - Miller parameter Fig. 5: Influence of tempering temperature on hardness and residual austenite content HIGH TEMPERATURE HEAT TREATMENT If this procedure is standardized, slow cooling rate can be used after initial solidification of the roll. Rolls produced in this way may have in the as-cast condition a hardness as low as 50 Shore with a pearlitic structure and can be premachined in such condition. After that, the specified hardness can be obtained by reaustenizing the roll at high temperature followed by a controlled cooling up to room temperature. The controlled cooling allows a good control of the precipitation of secondary carbides in the temperature range of 800 to 1050°c. Using that procedure, the roll metallurgist can produce and control the microstructure. After that quenching or controlled cooling, the roll is tempered to the hardness required. High temperature heat treatments produce a more uniform matrix homogeneity and structure than that which can be produced by controlled cooling alone. This is due to the fact that the macrosegregation phenomena cannot be neglected in those alloys. In case of rather fast solidification as in a spun-cast roll, we observe a higher chromium content in the core of the austenite cells than close to the eutectic carbides as shown on figure 6. It is clear that the diffusion connected to a high temperature heat treatment in the 1000/1050 °C range allows to homogenize the gradient of the elements in the austenite cells.
Fig. 6: Distribution of elements (Cr, C, Si) across a dendrite (15) Such heat treatment operations are rather expensive due to the slow heating rates that are required for massive metal sections such as rolls. They give however the best control of the hardness and of the component of the microstructure. ALLOYING ELEMENTS ADDITION (12, 14, 15) There are many metallurgical considerations in the selection and amount of the alloying elements. They interfere with each others. ·Chromium carbide composition, secondary carbide precipitation, and precipitation of special carbides during solidification (MC, M2C, …) ·Delaying of the pearlitic transformation ·Effect on the martensitic transformation temperature The formulation of balanced compositions for martensitic white iron casting should have the following major objectives. ·A sufficient alloy content to suppress the formation of pearlite in a given casting section size and cooling rate ·A sufficient quantity of carbide-stabilising elements to ensure freedom from graphite in the structure. ·A high carbon content to reach optimum abrasion resistance. ·A low silicon content to reduce the tendency for graphite formation and/or pearlite. ·Adjustment of alloy composition for controlling of the retained austenite . ·Addition of alloying elements to increase the hardness of the carbide phase. ·Addition of alloying elements to increase the toughness of carbides by modifying their composition, shape and distribution of the carbide phase. Molybdenum It is usually added in the range 0.5 to 4%. It has no effect on the liquidus surface ,and only the peritectic line d%gslightly moved to the low chromium content. The solidification sequence is unaffected but it lead tois the formation of small patches of eutectic Mo2C carbides for the low Cr/C ratio alloy(Cr/C=5) and to M6C carbides for the high Cr/C ratio alloys(Cr/C=10). Molybdenum is partitioned between Mo2C (50%), M7C3
(25%) and the matrix (25%). It however does have a significant influence neither on the total amount of carbides nor on the hardness of the austenitic matrix. In the as-cast condition, Molybdenum stabilises the austenite. It means that for given chromium content, pearlite or other transformation product can be suppressed at higher carbon contents. This effect is of considerable interest, as pearlite requires a high temperature close to solidus to be fully reaustenised during further heat treatment. On a commercial scale, such a treatment is difficult to carry out. During heat treatment, Mo will also inhibits the pearlitic transformation (5). Vanadium Vanadium is a strong carbide-forming element. In amount of 0.1 to 0.5 %, it is claimed by some authors to refines the as-cast structure and minimises columnar grain structure. It combines with iron to form both primary and secondary carbides during solidification thus lowering the carbon content of the matrix. Solidification proceeds with austenite, eutecticg%VC and finallyg%M7c3.The amount of vanadium carbides (MC) increases with the V/C ratio, however a certain quantity of vanadium is necessary , for a given carbon content , for the vanadium carbide to appear. Its composition is around 62-74% V, 9-11% Cr, 1.5-4.8% Fe,0-12% Mo. Like molybdenum, vanadium does influence neither the morphology of M7C3nor the total amount of carbides. The Vanadium content of the matrix increases with the V/C ratio. Vanadium improves the pearlitic hardenability and reduces the quantity of retained austenite, but is detrimental to the hardness in the as-quenched condition, the tempering strength and the hardness corresponding to a zero austenite percentage after tempering. This effect can be compensated by molybdenum in low Cr/C ratio but not in high Cr/C ratio alloys where it is very pronounced. The hardness increases with the vanadium content for a same Ms temperature. By raising the martensite transformation temperature, it leads to fully transformed structures in the as-cast conditions. Copper - Nickel Copper is frequently added to high chromium white cast irons because it is particularly effective in slowing down the austenite transformation rate. It thus allows martensite to form even in large sections castings. Copper is entirely located in the matrix. One disadvantage is its stabilising effect on austenite leading to excessive amount of retained austenite limiting its amount to 1 to 1.2%. Its effect is similar but less effective than nickel but is preferred because cheaper. Minor elements (Mn, Si) Manganese, like copper, nickel and molybdenum delay the appearance of pearlite in the structure. On the opposite, silicon accelerates its appearance. Silicon like copper is entirely located in the matrix while manganese is equally distributed between the matrix and the M7C3favours a highcarbides. Manganese amount of retained austenite while silicon contributes to its destabilisation. Minor elements (Mn,Si) have a very limited effect on the liquidus temperature and the solidification sequence remains unchanged.
MECHANICAL AND THERMAL PROPERTIES Table II gives a comparison of the main properties of indefinite iron, adamite and high chromium iron and steel (11, 16, 17). Table II Mechanical and Physical Properties IC Adamite Hi Cr iron Hi Cr steel Tensile strenght (MPa) 350 - 500 700 – 900 700 - 800 700- 800 Elongation (%) 0.2 - 0.3 0.4 - 1.0 0.4 - 0.6 0.4 - 1.0 Compression strength (MPa) 200 - 250 220 – 270 230 - 280 250 - 300 3 Elastic modulus ( 10 MPa) 180 - 190 200 – 210 220 - 230 200 - 210 -6 Thermal expansion coefficient (10 /°C) 13.0 - 13.4 12.5 - 13.0 12.7 - 13.2 11.5 - 12.0 2 Thermal diffusivity (cm /sec) 0.030 - 0.040 0.065 - 0.070 0.020 - 0.025 0.020 - 0.025 High chromium grades have an ultimate tensile strength twice higher than indefinite chill grade. It explains their very high performance as concern the fire crazing resistance. Fire crazing is recognised as being the most important deterioration phenomena in those early stands of Hot Strip Mills, which undergo high thermal solicitations. The low thermal properties of high chromium iron and steel in comparison of indefinite and adamite demonstrate the fact that those rolls are somewhat prone to damage in case of mill cobbles and stickers. Steeper temperature gradients are created in the roll shell. For that reason, most of the mill adopting high chromium grade improved their cooling practice, mainly by an increase of the cooling water amount (11). APPLICATION AND FUTURE DEVELOPMENTS Chromium alloyed rolls are mainly used in the roughing and early finishing stands of Hot Strip Mills and Compact Strip Process (CSP). Many heavy plate mills have also adopted that grade in substitute or in association with the ICDP grade. Some attempts were done to use high chromium rolls in the last finishing stands. Due to the lack of the formation of an oxide layer connected with the strong oxidation resistance of high chromium alloys, the rolls were prone to sticking and led to catastrophic mill incident. In the last finishing stands, high chromium iron rolls are presently used only for the rolling of checkered and corrugated plates. In the late seventies and early eighties, a lot of trials have been done in tandem cold mills for sheet and tin plate rolling. The purpose was to substitute as work rolls material a high chromium iron to forged steel. Due to the large amount of high chrome carbide in the microstructure, the wear resistance was very high giving output equal to twice and more of those of forged steel rolls. However, problems connected to the low thermal conductivity of high chromium iron and to a lack of cleanliness of the rolled sheet stopped almost all the use of that new grade in cold tandem mills. Actually, in the cold rolling area, high chromium iron rolls are used only in : ·First stand in tandem mill for tin plate ·2-high hot skin pass
·Back up rolls for 4-high skin pass In Japan, high chromium iron rolls are no more used in the early stands of hot strip mills and have been substituted by the high-speed steel rolls. But despite that that new development, most of the European and American hot mill are still using high chromium rolls. Presently, semi-HSS rolls are giving very high performance in roughing stand of some mills. Following metallurgical definition, it is possible to argue that new grade belong to the chrome steel family by its chromium content CONCLUSIONS The main advantages of the high chromium alloys are: ·The shape of the eutectic which favours a high toughness. ·The high hardness of the M7C3carbides which increases abrasion resistance. ·The hardenability which allows to achieve fully martensittic structure even in large sections when necessary. ·austenite allowing to achieve as-cast fully austenitic structure or to apply a thermalThe stability of treatment avoiding quenching. ·A high resistance to softening during tempering permitting high tempering temperature to achieve a good toughness and temperature resistance. ·The relatively low cost of chromium. All those advantages explain the success of the use of high chromium alloys for the manufacturing of rolling mill rolls. REFERENCES 1.Referenced in : A.B. Kinzel and W. Crafts, "The Alloys of Iron and Chromium", Low-Chromium Alloys, Mc Graw-Hill (1937) Vol. I 2.W. Tofaute, A. Sponheuer and H. Bennek, "Umwandlungs-, Härtungs- unb Anlassvorgänge in Stälen mit Gehalten bis 1% C und bis 12%Cr", Archiv für das Eisenhüttenwesen, 8, 1934-35, pp. 499-506 3.W. Tofaute, C. Küttner and A. Büttinghaus, “Das System Eisen-Chrom-Chromkarbid Cr7C3– Zementit”, Archiv für das Eisenhüttenwesen, 9, 1935-36, pp 607-617 5. F. Maratray and R. Usseglio-Nanot, "Factor Affecting the Structure of Chromium-Molybdenum White Iron", Climax Molybdenum S.A. Paris publication, 1970 6. F. Maratray, "Choice of Appropriate Compositions for Chromium-Molybdenum White Iron", AFS Transactions, 79, 1971, pp. 121-124. 7. J. Dodd, "High Chromium-Molybdenum Alloy Iron Rolls", Climax Molybdenum Company publication. 8. V.T. Zadan et al, translated from russian, Metallurg, 10, 1970, pp 30-32 9. R.S. Jackson, "The austenite liquidus surface and constitutional diagram for the Fe-Cr-C metastable system”, Journal of the Iron and Steel Institute, 208, 1970, pp. 163-167
