Materials Transactions, Vol. 47, No. 1 (2006) pp. 72 to 81 #2006 The Japan Institute of Metals

Effect of Alloying Elements on Heat Treatment Behavior of Hypoeutectic High Cast

Sudsakorn Inthidech1;*, Prasonk Sricharoenchai1 and Yasuhiro Matsubara2

1Department of Metallurgical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok10330, Thailand 2Department of Metallurgical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok10330, Thailand

The relationship between and volume fraction of retained austenite (V ) was investigated in heat-treated 16 mass% and 26 mass%Cr hypoeutectic cast with and without addition of a third alloying element of Ni, Cu, Mo and V. In as-hardened state, hardness changed remarkably depending on the V . Overall, Ni and Cu decreased hardness but Mo increased it. Hardness increased in 16 mass%Cr but decreased in 26 mass%Cr cast iron by V addition. The V increased with Ni, Cu and Mo addition but diminished with V addition in 16 mass%Cr cast iron. In 26 mass%Cr cast iron, Ni and Mo increased the V but Cu and V reduced it. Higher austenitization caused more V . Curves of tempered hardness showed an evident secondary due to precipitation of special carbides and transformation of destabilized austenite into martensite. High tempered hardness was obtained in the specimens with high V in as-hardened state. Maximum tempered hardness (HTmax) was obtained when V was less than 20% and it increased with an increase in Mo content. The HTmax slightly increased with V content in 16 mass%Cr cast iron and decreased in 26 mass%Cr cast iron. Ni and Cu did not show significant effects on HTmax. The highest value of HTmax was obtained in both series of cast irons containing Mo.

(Received September 5, 2005; Accepted November 9, 2005; Published January 15, 2006) Keywords: high chromium cast iron, alloying element, heat treatment, hardness, volume fraction of retained austenite

1. Introduction cooling rate.4) Once the high chromium cast iron has solidified, the High chromium cast irons, which contain 12 to carbide morphology can be little modified by any means 30 mass%Cr and 1.8 to 3.6 mass%C are well known as except by plastic deformation. However, the matrix structure abrasion wear resistant materials. It is said that their can be widely changed by heat treatment. In the practical use microstructures consisting of hard eutectic carbides of of high chromium cast irons, heat treatment has been (Cr,Fe)7C3 or M7C3 and strong supportive matrix provide conducted in order to achieve the optimal combination of excellent wear resistance. Consequently, high chromium cast hardness and quantity of retained austenite for abrasion wear irons have been widely used for parts or components in the resistance and other mechanical properties. In particular, the various industries. Their main and typical applications are for secondary carbides precipitated during destabilization heat mill rolls in the -making industry, and for rollers, treatment and the carbides formed during subsequent temper- tables and liners of pulverizing mills in cement, mining and ing plays an important role for the wear resistance and thermal power plant industries. mechanical properties. It is said that the maximum macro- The high wear performance of high chromium cast iron hardness obtained after is at most 800 HV30 in depends on both the amount and the type of eutectic carbide high chromium cast iron without any addition5) and that and on the matrix structure. Many papers report that the wear the hardness in eutectic high chromium cast iron can be resistance increases with an increase in volume fraction of increased by addition of Mo.6) In the case of hypoeutectic eutectic carbide under abrasive wear conditions.1,2) However, cast iron, some alloying elements which improve the hard- high volume fraction of eutectic carbide deteriorates the enability and simultaneously promote the formation of toughness which is a critical factor, particularly, when the secondary carbides with higher hardness than chromium high chromium cast iron is used for construction parts. In carbides should be added. In spite of many researches on high order to get higher toughness, the volume fraction of eutectic chromium cast irons containing alloying elements, system- carbides must be limited and also coarse primary carbides be atic and detailed investigations of the effects of alloying avoided. Hence, high chromium cast iron with hypoeutectic elements on the behavior of hardness and retained austenite composition is widely used. during heat treatment are very limited.7,8) Solidification of high chromium cast iron has been studied In this study, therefore, hypoeutectic high chromium cast by many researchers1–4) and it is found that austenite () irons, to which Ni or Cu is separately added to improve dendrites solidify first in hypoeutectic cast iron, followed by mainly hardenability as well as Mo or V to promote a eutectic reaction of L !ð þ M7C3Þ. The eutectic is found precipitation hardening, were employed. The investigation to grow with a cellular interface and to solidify as a colony was focused on the variation of hardness and the volume structure.4) The carbide morphology in the eutectic structure fraction of retained austenite including their correlation with depends on the type and the amount of carbide forming the heat treatment conditions. elements, namely Cr, Mo, V and carbon content. The eutectic morphology, i.e., the sizes of eutectic colony and individual 2. Experimental Procedure carbide particle can be controlled by solidification rate or 2.1 Preparation of test specimens *Graduate Student, Chulalongkorn University 16 mass%Cr and 26 mass%Cr hypoeutectic cast irons Effect of Alloying Elements on Heat Treatment Behavior of Hypoeutectic High Chromium Cast Iron 73

Table 1 Chemical composition of test specimens.

Element (mass%) Number CCrSiMnNiCuMoV No. 1 3.01 16.48 0.62 0.78 — — — — No. 2 2.90 16.23 0.51 0.53 1.21 — — No. 3 2.95 16.00 0.55 0.56 2.14 — — — No. 4 2.94 15.84 0.54 0.56 — 1.00 — — No. 5 2.96 15.90 0.54 0.53 — 2.02 — — No. 6 2.97 16.12 0.52 0.50 — — 0.96 — No. 7 2.97 15.93 0.51 0.51 — — 2.03 — No. 8 3.05 15.82 0.55 0.51 — — 3.05 — No. 9 2.96 16.13 0.50 0.51 — — — 1.00 No. 10 3.13 16.08 0.53 0.53 — — — 2.10 No. 11 3.06 16.06 0.47 0.50 — — — 3.05 No. 12 2.65 25.56 0.37 0.51 — — — — No. 13 2.65 26.16 0.48 0.48 1.18 — — — No. 14 2.64 25.91 0.47 0.51 2.06 — — — No. 15 2.62 26.31 0.50 0.50 — 1.03 — — No. 16 2.68 26.04 0.51 0.51 — 2.10 — — No. 17 2.66 25.84 0.48 0.50 — — 0.98 — No. 18 2.61 25.90 0.50 0.50 — — 1.95 — No. 19 2.64 26.40 0.49 0.47 — — 2.96 — No. 20 2.61 25.97 0.51 0.50 — — — 1.05 No. 21 2.64 25.91 0.51 0.51 — — — 2.02 No. 22 2.71 26.46 0.52 0.51 — — — 3.02

without alloy addition and with a separate addition of Ni or tion were (200) and (220) planes for ferrite () or martensite Cu up to 2 mass% and Mo or V up to 3 mass% were produced and (220) and (311) planes for austenite (). using a 30 kg-capacity high frequency with alumina lining. Raw materials such as mild steel, , 3. Experimental Results and Discussions ferro-alloys and pure metals were used as charge materials. The charge materials were melted down and superheated up 3.1 Microstructure of test specimen to 1853 K. After holding at the , each melt was Typical as-cast microstructures of 16 mass%Cr cast irons poured from 1773–1793 K into preheated CO2 mold with a with and without alloying elements are shown in Fig. 1. The cavity size of 25 mm in diameter and 65 mm in length, and microstructures consist of primary austenite dendrites and the surface of the top riser was immediately covered with dry eutectics structures. The microstructure of the alloy-free cast exothermic powder to prevent the riser from cooling. The iron consists of pearlitic matrix with rod-like and massive chemical compositions of the test bars are shown in Table 1. eutectic M7C3 carbides. The ( þ M7C3) eutectic shows a The bars were sectioned by a wire-cut machine to obtain colony morphology that contains fine carbides in the central disk-shaped test pieces, 7 mm in thickness. region and coarse carbides at the boundary region. In the cast irons with alloying element, the carbide morphology changes 2.2 Heat treatment little whereas the matrix remarkably changes depending on First of all, the as-cast specimens were annealed for the kind and the amount of alloying element. The matrix in homogenizing by holding at 1173 K for 10.8 ks and then the cast irons alloyed with Ni or Mo is austenitic and that in cooled in a furnace. The annealed specimens were then the cast iron with V is pearlitic. A major part of matrix in austenitized at 1273 and 1323 K for 5.4 ks and hardened by the cast iron with 1 mass%Cu is pearlitic and that with forced air cooling with approximate cooling rate of 12 K/s. 2 mass%Cu is mostly austenitic. The hardened specimens were tempered at 50 K intervals As for the microstructures of 26 mass%Cr cast irons, all the between 573 and 873 K for 7.2 ks. matrices, with and without alloying element, were austenitic, and possibly containing some martensite. It was also found 2.3 Measurement of hardness and retained austenite that the eutectic colony size of 26 mass%Cr cast iron was The measurement of macrohardness was carried out by a smaller and the size of eutectic carbide particles was much Vickers hardness tester with a load of 300 N (30 kgf). More smaller than those of 16 mass%Cr cast iron. than five indentations were taken at random and the measured values were averaged. The volume fraction of retained 3.2 Effect of alloying elements on hardness and volume austenite (V ) was obtained by an X-ray diffraction method fraction of retained austenite (V ) using a special goniometer with automatic rotating and tilting 3.2.1 As-hardened state sample stage. The diffraction peaks used for V determina- Macrohardness and V were measured in all the as- 74 S. Inthidech, P. Sricharoenchai and Y. Matsubara

150 µm

Alloy-free

% 1% 2% Alloy

Ni

Cu

Mo

V

Fig. 1 As-cast microstructures of 16 mass%Cr cast irons free of and with third alloying element. hardened specimens. The hardness depends on the type and increase in Ni content regardless of the austenitizing temper- the volume fraction of carbide and matrix structure, and the atures. Cu shows the same effect except for the 16 mass%Cr matrix hardness is directly related to the V in hypoeutectic specimens austenitized at 1273 K. An addition of Mo leads to cast iron. Generally, greater amounts of carbide leads to an an increase in the hardness in all specimens. As V content increase in hardness but greater amount of retained austenite increases, the hardness rises in 16 mass%Cr specimens but which is dependent on the type and amount of added alloying unexpectedly reduces in 26 mass%Cr cast irons. The highest element produces a decrease in hardness. The relationship hardness is obtained in the 16 mass%Cr cast iron containing between alloying elements and macrohardness is shown in V and 26 mass%Cr cast iron with Mo. Fig. 2. In all specimens, the hardness is decreased with an The macrohardness measured is closely related to the Effect of Alloying Elements on Heat Treatment Behavior of Hypoeutectic High Chromium Cast Iron 75

16 mass%Cr cast iron 26 mass%Cr cast iron

(a)Austenitized at 1273 K

Alloy (mass%)

Fig. 2 Effect of alloying elements on macrohardness in as-hardened specimens with 16 mass% and 26 mass%Cr.

16 mass% Cr cast iron 26 mass% Cr cast iron

((mass%)

Fig. 3 Effect of alloying elements on volume fraction of retained austenite (V ) in as-hardened specimens with 16 mass%Cr and 26 mass%Cr. matrix structure, particularly, volume fraction of retained content is higher in 16 mass%Cr cast iron than those in austenite. When the concentrations of Cr and C in austenite 26 mass%Cr cast iron. It is accepted that in any austenitizing are compared between 16 mass%Cr and 26 mass%Cr cast condition, Ni and Cu which dissolve entirely in the austenite irons, it is generally said that Cr content is lower and C depressed Ms temperature and as a result more austenite was 76 S. Inthidech, P. Sricharoenchai and Y. Matsubara

Alloy-free

Ni Cu

1 mass% 1 mass%

2 mass% 2 mass%

Fig. 4 Relationship between hardness, volume fraction of retained austenite (V ) and tempering temperature in 16 mass%Cr specimens free of alloying element and with Ni or Cu. retained. On the other hand, Mo and V are strong carbide as follows: formers and are consumed in the formation of carbides. When specimens were hardened, the cooling rate is faster However, even when V, a very strong carbide former, is than the critical cooling rate of their pearlite transformation. added to 16 mass%Cr cast iron, sufficient carbon remains in Therefore, the V could depend on the bainite transformation the matrix to form martensite. In the case of 26 mass%Cr cast which occurs at lower temperature. When the bainite iron, C in austenite combines with V reducing the C content transformations are compared in high chromium cast irons in the matrix and allowing transformation to pearlite. with similar chemical composition, the nose time in 2.6% C– The V is increased when austenitizing temperature is 27% Cr cast iron is located at short time side than that in 2.9% 9) high. The effect of alloy content on the V is shown in Fig. 3. C–17% Cr–0.5% Mo cast iron. Since the cooling rate of this In 16 mass%Cr cast irons, the V increases roughly in experiment is near the bainite transformation of 16% Cr cast proportion to Ni, Cu and Mo contents but gradually decreases iron, the effect of Cu content on the bainite transformation with an increase in V content. In the case of 26 mass%Cr cast must be more in this 16% Cr cast iron than in 26% Cr cast irons, the behavior of the V is shown similar to that of iron and the amount of bainite transformation could be 16 mass%Cr cast irons. In 26% Cr cast iron, however, the V reduced with an increase in Cu content. Resultantly, the V seems to decrease very slightly or it can be regarded that it is increases. From the relationship between the positions of almost same. At the same alloy content, the V values in bainite transformation, the specimens of 26% Cr cast iron are 16 mass%Cr cast irons are higher than those in 26 mass%Cr participated more in the bainite transformation. Even if the cast irons. The reason why Cu increases the V in 16% Cr cast bainite transformation is shifted by increasing the Cu content iron but changes it little in 26% Cr cast iron can be explained up to 2% in 26% Cr cast iron, the amount of bainite Effect of Alloying Elements on Heat Treatment Behavior of Hypoeutectic High Chromium Cast Iron 77

Mo V

1 mass% 1 mass%

2 mass% 2 mass%

3 mass% 3 mass%

Fig. 5 Relationship between hardness, volume fraction of retained austenite (V ) and tempering temperature in 16 mass%Cr specimens with Mo or V. transformation is not influenced so much by Cu content. Due 3.2.2 Tempered state to these reasons, the V change little by increasing Cu content It is reported that eutectic high chromium cast iron with the in 26% Cr specimens. carbide forming elements exhibits precipitation hardening Here, the effects of V and Mo on as-hardened hardness can during heat treatment and the highest hardness is 900 HV30 6) be explained by referring to the V . In 16 mass%Cr cast iron, in 26 mass%Cr cast iron with Mo. The effect of precip- V containing specimens show the highest hardness because itation hardening in hypoeutectic irons must be greater than as-hardened matrix contained small amounts of V and large in irons with eutectic composition because of the greater portions of martensite. In the 26 mass%Cr cast irons, on the portion of matrix area to be precipitated. During tempering, contrary, the hardness decreases with V. This reason is the retained austenite in as-hardened state is decomposed by explained by the fact that although the as-hardened matrix the precipitation of carbides and the rest of the austenite contains very low quantity of V less than 5%, the remainder transforms into martensite during cooling after holding at the was pearlite nor martensite. Since the amount of pearlite in tempering temperature. Martensite existing in as-hardened specimen with Mo is less, on the other hand, the hardness state is simultaneously tempered and secondary carbides are depends mainly on the V . It is reported that when the also formed by so-called carbide reaction. hardness is related to the V , the V which provides the The relationships between hardness, V and tempering highest hardness in alloyed white cast irons is said to be 20 to temperature of alloyed 16 mass%Cr and 26 mass%Cr cast 10) 30%. The V at the maximum hardness in this experiment irons are shown together with alloy-free cast irons in Figs. 4 is 15 to 20% which is in very close agreement. to 7, respectively. In every case, the tempered hardness 78 S. Inthidech, P. Sricharoenchai and Y. Matsubara

Alloy-free

Ni Cu

1 mass% 1 mass%

2 mass% 2 mass%

Fig. 6 Relationship between hardness, volume fraction of retained austenite (V ) and tempering temperature in 26 mass%Cr cast irons free of alloying element and with Ni or Cu.

curves show evidence of secondary hardening. The degree of 748 and 800 K, except for the case of Mo addition, regardless secondary hardening, which is defined as the difference in of austenitizing temperature and alloy content. The temper- hardness between the maximum tempered hardness (HTmax) ing that the V becomes 0% are about 773 to and the hardness at which the secondary hardening begins, is 873 K in Mo added specimens. In both series of the cast irons, larger in alloyed cast iron than that in alloy-free cast iron. It is the HTmax is obtained when the specimens are tempered at seen that the degree increases as both the alloy content and 748 to 823 K where the V is reduced to less than 20%. The the austenitizing temperature rise, and that the increase seems HTmax is high in the case of high austenitizing temperature to be related to the V value in as-hardened state, the higher and the temperature to obtain the HTmax shifts to the high the V , the greater the degree of secondary hardening. From temperature side. This reason could be that a large amount of the tempering curves of the specimens with 2 mass% alloying retained austenite in as-hardened state needs high tempering element in both 16 mass%Cr and 26 mass%Cr cast irons, the temperature to be decomposed in an optimum condition for a degree of secondary hardening is calculated and the results fixed holding time. are shown in Table 2. The specimen with Ni shows the As mentioned above, the V in as-hardened state is highest degree of secondary hardening followed by Cu, Mo similarly related to the HTmax under the same tempering and V, respectively. According to the tempering curves condition. Here, the HTmax is related to the V in as-hardened shown in Figs. 4 to 7, the V begins to decrease abruptly or state as shown in Fig. 8(a) for 16 mass%Cr and Fig. 8(b) for sometimes gradually from the tempering temperature of 26 mass%Cr cast irons. In the case of 16 mass%Cr cast iron, 673 K and to be zero at the tempering temperatures between even when the V increases, the HTmax does not change by an Effect of Alloying Elements on Heat Treatment Behavior of Hypoeutectic High Chromium Cast Iron 79

Mo V

1 mass% 1 mass%

2 mass% 2 mass%

3 mass% 3 mass%

Fig. 7 Relationship between hardness, volume fraction of retained austenite (V ) and tempering temperature in 26 mass%Cr specimens with Mo or V.

Table 2 Degree of precipitation hardening in specimens with 2 mass% less than 10%. In this range, the hardness could be influenced alloying element. not only by V but also by precipitated carbides formed from V V Alloying Degree of precipitation hardening (HV30) martensite during tempering. Over 10% , the effect of on the H is significant in the specimens with Mo and V. element 16 mass%Cr 26 mass%Cr Tmax The effects of alloying elements on HTmax are shown in Ni 181 87 Fig. 9. At the same alloy content, the HTmax is high in the Cu 175 85 specimens hardened from higher austenitizing temperature. Mo 158 66 In 16 mass%Cr cast iron, the HTmax increases remarkably as V3548Mo content increases and slightly with V content but does not change by Ni and Cu contents. In 26 mass%Cr cast irons, the HTmax increases largely with an increase in Mo content but it increase in Ni and Cu content but it is gradually increased by is decreased by Ni and V contents. Cu has little effect on increasing Mo and V contents. In the 26 mass%Cr cast iron, the HTmax. The highest values of HTmax, 875 HV30 in 16 the HTmax increases with an increase in the V except for the mass%Cr and 885 HV30 in 26 mass%Cr cast irons, were cast iron with Ni in which HTmax does not change. The HTmax obtained in the irons with 3% Mo. In spite of greater of the specimens containing V abruptly increases in the small secondary hardening, highest HTmax in hypoeutectic cast iron range of V from 2 to 5%. So, V in retained austenite may with 3% Mo is a little lower than that in eutectic cast iron promote the formation of its secondary special carbide. The with 3% Mo. This is due to the difference in the volume HTmax of 26 mass%Cr specimens with Cu rises in the range fraction of eutectic carbide (Vc) between them, Vc of the 80 S. Inthidech, P. Sricharoenchai and Y. Matsubara

Fig. 8 Relationship between maximum tempered hardness (HTmax) and volume fraction of retained austenite (V ) in the as-hardened state. (a): 16 mass%Cr, (b): 26 mass%Cr cast irons.

16 mass% Cr 26 mass% Cr

Fig. 9 Effect of alloying elements on maximum tempered hardness (HTmax) in 16 mass% and 26 mass%Cr cast irons. (a) and (b) for 16 mass%Cr, (c) and (d) for 26 mass%Cr. former is 31.5% and that of the latter is 36.4%. Ni and Cu do both the temperature of eutectoid transformation and a 12) not contribute the increase in the HTmax, and therefore, such critical cooling rate of pearlite transformation. Conse- elements should be added for the improvement of harden- quently, V in austenite promotes pearlite formation and then ability. Mo contributes more to increase the hardness because less austenite with low V content is retained in as-hardened the large amount of retained austenite in as-hardened state state. In order to make V provides its full effect on an increase contributed greatly to the secondary precipitation of M6C in hardness, extremely high austenitizing temperature may be carbides with much higher hardness than chromium carbides introduced for more and more dissolution of V in the of M23C6, and as a result, the matrix hardness is increased. In austenite. spite of the fact that V is strong carbide former and carbide has extremely high hardness, the high hardness could 4. Conclusions not be obtained in the specimens with V by this heat treatment, and V rather lowers the tempered hardness in 26% General heat treatments, , hardening and temper- Cr specimens. This is because the cast iron with V produces ing, were conducted on 16 mass%Cr and 26 mass%Cr less retained austenite in as-hardened state since V shifts the hypoeutectic cast irons with and without the third alloying pearlite transformation to short time side.11) Also V increases element of Ni, Cu, Mo and V, each, and the relationship Effect of Alloying Elements on Heat Treatment Behavior of Hypoeutectic High Chromium Cast Iron 81 among alloying elements, hardness and volume fraction of V increased it slightly in 16 mass%Cr cast iron but retained austenite was clarified. The conclusions drawn from decreased it in 26 mass%Cr cast iron. Ni and Cu did not the experimental results are as followed. show a significant effect on the HTmax. (6) The highest values of HTmax, 875 HV30 in 16 mass%Cr 4.1 In as-hardened state and 885 HV30 in 26 mass%Cr cast irons with Mo, were (1) Hardness changed remarkably depending on the vol- obtained respectively. ume fraction of retained austenite (V ) which was closely related to the kind and the amount of alloying Acknowledgements element. The hardness decreased with an increase in Ni and Cu contents in both series of cast irons. Mo The authors would like to note with appreciation that increased the hardness overall. V increased the hardness financial support from the Thailand Research Fund through in 16 mass%Cr cast irons but reduced hardness in 26 the Royal Golden Jubilee Ph. D. Program (Grant No. PHD/ mass%Cr cast irons. 0165/2547) to student’s initials and advisor’s initials is (2) The V increased with an increase in Ni, Cu and Mo acknowledged. contents and it was reduced by increasing V content in 16 mass%Cr irons. 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