This paper present studies on the effects of quenching temperature, tempering temperature and time on the hardness of three die steels of similar carbon contents but having increasing chromium percentages. While tempering in the range of 150°C to 400°C it was observed that hardness decreased as quenching temperature was raised. All the steels showed secondary hardening phenomenon in relation to the variables mentioned above. It was also observed that on tempering the steels above 450°C, after prior quenching from increasing temperatures, hardness gradually increased upto the quenching temperature of 1100°C, beyond which followed a decrease in hardness. This phenomenon was the decrease in hardness. This phenomenon was the same at all tempering temperatures upto 601°C, but the increase in hardness from low quenching temperatures to the maximum quenching temperature was less at 450°C than at the higher temperatures and attained a maximum at 600°C secondary hardening decreased with the increase of quenching temperatur

Chakravarty, S P

Dubey, R K

Gupte, P K

English

eprints@NML

Tempering of die-steels based on indigenous materials R. K. DUBEY, S. P. CHAKRAVARTY and P. K. GUPTE This paper present studies on the effects of quenching tempera-ture, tempering temperature and time on the hardness of three die steels of similar carbon contents but having increasing chromium percentages. While tempering in the range of 150°C to 400°C it was observed that hardness decreased as quenching temperature was raised. All the steels showed secondary hardening phenomenon in relation to the variables mentioned above. It was also observed that on tempering the steels above 450°C, after prior quenching from increasing temperatures, hardness gradually increased up to the quenching temperature of 1100°C, beyond which followed a decrease in hardness. This phenomenon was the same at al tem-pering temperatures upto 601° C, but the increase in hardness from low quenching temperatures to the maximum quenching tem-perature was less at 450°C than at the higher temperatures and attained a maximum at 600°C. At 650°C secondary hardening decreased with the increase of quenching temperature. T. 	 HE development of die steels based on indigenous materials is closely associated with the phenomenon of decomposition of martensite and retained austenite and formation and distribution of carbides in a tem-pered steel. The end use of die-steel is primarily a question of compromise between hardness and toughness charac-terised and developed by proper heat-treatment. It is therefore necessary to examine the changes that occur during heating, quenching and tempering. Extensive studies have been made on the tempering behaviour of tool steels by several investigators 1-3 who established that steels are normally characterized in the quenched state by substantial amounts of retained austenite in addition to untemperd martensite and excess carbide and who also observed the phenomenon of decompo-sition of retained austenite during cooling from the tem-pering temperature. It is known that the rate of de-composition of retained austenite is not the same throughout the tempering range ; the isothermal reacti-vity of retained austenite is very prompt in the lower range of tempering temperature and rather slow as tempering temperature is increased above 425°C. As a result more austenite is available for transformation during cooling and it has probably a structure composed of bainite or tetragonal martensite or both depending on the conditioning of retained austenite. The effects of quenching temperature, tempering tem-perature and time on hardness have been studied on Messrs R. K. Dubey & P. K. Gupte, Scientists & Mr. S. P. Chakravarty, Senior Laboratory Assistant. National Metallurgi-cal Laboratory, Jamshedpur.  three die steels of similar carbon content with increas-ing chromium percentages. Secondary hardening pheno-mena were observed in steels containing chromium in relation to these three variables. Experimental procedure The investigation was carried out on three commer-cially important die-steels based on indigenous mate-rials such as chromium and vanadium. These steels were produced in the laboratory in a 25-lb basic lined electric induction furnace. The cast ingots were forged to 2" sq. bars after proper annealing. The steel bars were subjected to a long homogeni-zing treatment for uniform dissemination of segregated atoms. Table I gives the chemical analysis of the steels investigated. TABLE 1 Chemical compositions of steels investigated (weight %) Steel C Si Mn Cr V Ti 	 ?vio NV Ds2 P13 0'37 1.03 1'58 0.14 Trace 0.10 Ds3 P09 0'51 0'88 2'46 0.11 Trace 0.09 --Ds, 1.01 0.32 0.75 4'82 0'27 Trace 0.15 — +1101CnO.N.M.••n•• The steel samples of 5/8" x 3/8" x e" size were surface ground on both the faces and bright nickel plated in order to prevent oxidation during heating at high temperatures. In each test, the 1" dia. tube furnace used for the austenitizing treatment provided a mild carbonaceous atmosphere in order to minimise the loss in surface carbon due to oxidation during heating of the specimen. Mineral oil was used for quenching and hardening throughout the experiments. After suitably heating the specimens to various austenitizing temperatures, they were quenched in oil and immediately after cooling to hand warm temperature they were re-heated to a series of temperatures between 150°C and 650°C. Salt-bath was used for lower temperature tempering upto 350'C, and lead-bath for higher temperature tempering. 26 	 NML Technical Journal 800 700 600 F, 500 400 300 00 200 300 400 500 600 TEMPERING TEMPERATURE °C 200 0 After tempering the specimens were generally cooled 	 goo in air. Effect of quenching temperature on hardness For obtaining maximum hardness in steel, the primary condition is that mainly carbon and less significantly the other alloying elements should completely go into austenitic solution so that on quenching, the break-down of austenite bears a maximum hardness retation-ship to the carbon content of the steel. The solubility of carbides, which depends on their nature and com-positions, varies in different compositions of steels and progressively increases as the austenitizing temperature is raised. As many as five quenching temperatures were used to study the effect of solution of carbides on hard-ness. The austenitizing temperatures were selected at 800`C, 850°C, 900°C, 950°C and 1000°C. Table H shows the effect of quenching temperature on hardness. TABLE 11 Effect of quenching temperature on hardness Hardness (VPN) Quenching           temperature °C 	 Steel DS2 Steel DS3 	 Steel DS, 800°C 880 832 850°C 877 880 870 900°C 823 817 905 950°C 817 771 840 1000°C 778 760 790 It is clear that as the quenching temperatures in-crease, steels DS2 and DS3 drop in hardness above 850°C. In steel DS4, the hardness reaches its maximum at the quenching temperature of 900°C. Above the quenching temperature of 900°C the amount of retained austenite was a little larger. At lower quenching tem-peratures hardness decreased as usual. All the steels showed a marked similarity in this behaviour. Effect of quenching media on the as-quenched hardness Three quenching media e. g. water, oil and air were chosen to study the effect of cooling rate on the degree of martensitic hardness as well as the amount of retained austenite. Table HI shows the changes in hardness values due to different cooling rates. Steels DS, and DS3 were quenched at 850°C and steel DS4 at 900°C. It is clear from this table that after quenching from the same austenitizing temperature, rapid cooling im-parts greater hardness than slow cooling. Such differen-ces in the hardness values were attributed to the fact that more austenite is retained in slower cooling rates.  1 Effect of tempering time on hardness of steel DS2  TABLE III Changes in hardness values with different cooling rates Quenching media Hardness (VPN) Steel DS2 Steel DS3 Steel DS, Water Oil Air 897 877 815 900 880 820 920 905 835 Effect of tempering time Experiments were conducted to study the effect of holding time on breakdown of retained austenite during tempering of steels DS2, DS3 and DS4. After quenching from the austenitizing temperature the specimens of steel DS2, DS3 and DS4 were tempered at temperatures between 150°C and 650°C for hour, 1 hour and 2 hours. The resulte are represented in Figs. 1, 2 and 3. 100 200 300 400 500 600 700 TEMPERING TErot 2 Effect of tempering time on hardness of steel DS2  August 1969 27 20 100 200 300 400 500 600 700 TEMPERING TFNP-ERATUREt 3 Effect of increasing tempering time on hardness of steel DS, 900 800 700 z 600 z 500 300 850°C 950°C (100° C I150°C 200 200 300 400 500 600 TEMPERING T EMPERATURE°C 5 Hardness developed in four specimens of steel DS3 quenched fr om 850°C, 950°C, 1100°C and 1150°C, after various reheats 700 600 z 500 z 240 300 700  70,0 300 400 500 600 741, TEMPERING 600 1500 z 0 tt 400 300 200 	  200 The curves show clearly that as tempering time is increased subsequent hardness decreases more or less uniformly. Steel DS3 showed a flattening in the tem-pering range of 350 to 400°C ; in steel DS4, this flat-tening tendency was found to prolong upto 500°C, while no appreciable flattening tendency was observed with steel DS,. Effect of quenching temperatures on tempering characteristics The hardness characteristics of steels tempered at vari-ous temperatures were studied and the results are summarized in Figs. 4, 5 and 6. Vickers Pyramid hardness readings were taken on each quenched and tempered specimen with due precautions to ensure accurate results. Figs. 4, 5 and 6 show that there was no secondary hardening until the tempering temperature reached 450°C and at this point a marked increase in hardness with increasing quenching temperature was observed, which predominated at all tempering tempera-tures upto 650°C. This observation held good upto the quenching temperature of 1100°C, after which there was a drop in hardness. Hardness increased steadily due to secondary hardening in relation to quenching temperature and tempering temperature and attained a maximum at the tempering temperature of 600°C, when the increase in steel DS3 and DS4 was as much as 95 and 150 VPN respectively. At 650°C, the increase in hard-ness was much less and might be regarded as virtually the upper limit of tempering temperature beyond which hardness decreased rapidly without secondary hardening effect. Similar effect of secondary hardening was observed in steel DS, also, but to a smaller degree, owing to the lower chromium content. Effect of duble tempering In order to obtain an idea of the effect of double tempering on these steels the samples DS,, DS3 and DS4 which were quenched and hardened from the austenitizing temperature of 950°C were tempered at 450°C, 550°C and 650°C for 1 hr and allowed to cool 4 Hardness developed in four specimens of steel DS, quenched from 850°C, 950°C, 1100°C and 1150°C after various reheats 200 300 400 500 600 700 TEMPERING TEMPERATURE °C 6 Hardness developed in steel DS4  quenched from 900°C, 950°C, 1050°C and 1100°C, after various reheats 900 800 -E700 600 z 0 T 500 40 300 	  100 28 	 NML Technical Journal n air until their temperature was handwarm. Imme-diately the same specimens were retempered at the previous tempering temperatures for further one hour. It was seen that the hardness had dropped appreciably indicating the absence of retained austenite after the specimen was tempered once. Discussion of results At lower ranges of tempering temperature, the quench-ed and hardened die steels have greater tendency to yield to softening, while on raising the tempering temperature above 425°C a sudden change in the precipitation mechanism in steel develops a secondary hardness, a vital change in the combined effect of strength and toughness during the process of tempering. The whole process of tempering mechanism can be broadly divided into four stages. During the first stage of tempering, tetragonal martensite decomposes into cubical symmetry and is characterized by a marked softening and a decreased volume. It has been found during tempering in the range of 150° to 400°C that hardness decreased as the quenching temperature was raised. This softening, i. e. decrease in hardness is perhaps due to the agglomeration of cementite partic-les rejected by the tetragonal martensite, without having a chance to precipitate as alloy carbides due to the low diffusion rate of alloying elements at such low tempering temperatures. The amount of retined austenite available for iso-thermal decomposition is also dependent on the hold-ing period for which the steel is tempered. At such low tempering range retained austenite decomposes partially into bainite for want of sufficient time (1 hr) for the transformation of retained austenite. With increasing quenching temperature there is greater occu-rence of retained austenite which causes a regular drop in hardness during the first stage of tempering of steels DS2, DS3 and DS,. In all the steels the second stage of tempering is characterized by the precipitation of alloy carbides from the retained austenite as the tempering tempera-ture is raised to 450°C and above. As a result there has been an increase in hardness as well as in resis-tance to softening. When tempering temperatures are varied in the range 450°C to 600°C the increase in hardness is neither the same nor is it regular. It was observed that on tempering the steels at 450°C after prior quenching  from increasing temperatures, hardness gradually in-creased upto the quenching temperature of 1100°C, after which it decreased. This phenomenon was cons-tant for all the tempering temperatures. But at the tempering temperature of 450°C the increase in hard-ness from low quenching temperature to maximum quenching temperature is less than at the higher tempering temperatures. This increase in hardness, which is due to the secondary hardening becomes maximum as the tempering temperature reaches 600°C. As the compositions of these steels differ only by their chromium content a lower value is obtained in steel DS2. The slight increase in hardness at 450° and 500°C is due to involvement of more alloy carbides precipitated from retained austenite and possibly does not account for the decomposition of retained austenite which is likely to be very sluggish or conditioned. The third stage of tempering attains maximum secon-dary hardness in a comparative study with the veriation of quenching temperature in relation to a particular tempering 	 temperature. At 550° and 600°C there was a rapid increase in hardness as quenching temperature varied. This increase is due to the decom-position of retained austenite which is greatly relieved of sluggishness by prior precipitation 	 of carbides and the transformation of austenite take place during cooling of the steel in air after tempering. At 650°C the secondary hardening decreased with the increase of quenching temperatures. This is followed by coagulation of cementite of tetragonal martensite with the alloying elements diffusing at a rapid rate. When quenched die steels are tempered at high tem-perature (above 650°C) they approach the annealed conditi on. Acknowledgements The authors are thankful to Shri P. I. A. Narayanan, Scientist-in-Charge, National Metallurgical Laboratory, for kind permission to publish this paper. References 1. Cohen, M. and Koh, P. K. "Tempering of high speed steel", Tr. A.S.M. Vol. 27, 1939, p. 1015. 2. Lenient B. S. and Cohen, M. "Tempering of two high carbon, high-vanadium high speed steels" Tr. A.S.M., Vol. 30, 1942, p. 1021. 3. Zmeskal 0. and Cohen, M. "The tempering of two high carbon, high chromium steels" Tr. A.S.M, Vol. 31, 1943, p. 380. August 1969 	 29 

Tempering of Die-steels based on indigenous materials

http://eprints.nmlindia.org/1999/3/26-29.PDF

Tempering of Die-steels based on indigenous materials

Abstract

Similar works

Full text

Available Versions

eprints@NML