NZ201260A - Heat-resistant cast steel - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £01 260 201260 v 3 t 1 Ccrojile&a f>;*c.hcai. ition Fiiad: '~P.'.$.£■ a i i I •■•••■•■•■■■■ Pt&'Hcalicn Dete: P.O. JscmrA Kg: , ... m m *ff£ is f* NEW ZEALAND PATENTS ACT, 1953 No: Date: COMPLETE SPECIFICATION HEAT RESISTANT CAST STEEL -H3 JUL ft&Z "V' ^iv>.----rr t We, KUBOTA LTD, a Japanese company, of 2-47, Shikitsuhigashi 1-chome, Naniwa-ku, Osaka, Japan hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: (followed by page -la-) 2 012 6 TITLE OF TH5 INVENTION heat nnciGTAHT oast steel BACKGROUND OF THE INVENTION The present invention relates to heat resistant 5 cast steel, and more particularly to heat resistant cast steel which essentially has the composition of austenitic cast steel containing Cr, Ni, Fd and W and which is excellen in creep fracture strength at high temperatures and in resistance to thermal impact or carburizing. 10 HK 40 which is a heat resistant cast steel containing Ni and Cr (25 Cr-20 Ni steel, see ASTD/i A 608) and HP materials (see ASTI'/i A 297 ) have "been used as materials for ethylene cracking tubes in the petrochemical industries. With the elevation of operating temperatures 15 in recent years, it has been required to improve the high-temperature characteristics of such materials. To meet this requirement, HP materials containing Nb and W or HP materials containing Nb, W and Mo have been developed and placed into use. However, with the recent tendency 20 toward severer operating conditions, it is desired to provide materials which are superior to such HP materials containing Nb ana W, or Nb, V/ ana Mo in respect of high-temperature creep fracture strength and resistance to thermal shock or carburizing. -1ft- sf) - SUMMARY OF THE INVENTION In view of the above demand, we have conducted intensive research on the influence of variously contained elements on the high-temperature characteristics of heat-5 resisting cast steel containing Cr, Ni, Nb and W as the essential components and found that the steel can be remarkably improved in high-temperature creep fracture strength and resistance to thermal shock and to carburizing by containing Ns B, Ti and Al therein, with further use 10 of Mo when desired. Thus this invention has been accomplished.

Stated specifically, the present invention provides a heat resistant cast steel containing about 0.3 to 0.6$ (by weight, the same as hereinafter) of C, up to 15 about 2.0$ of Si, up to about 2.0$ of Mn, about 20 to 30$ of Cr, about 30 to 40$ of Ni, about 0.3 "to 1.5'$ of Nb + Ta, about 0.5 to 3*0$ of W, about 0.04 to 0.15$ of N and about 0.0002 to 0.004$ of B, the steel also containing about 0.04 to 0.15$ of Ti and about 0.02 to 0.07$ of Al, or about 20 0.04 to 0.50$ of Ti and about 0.07 "to 0.50$ of Al, the steel further containing about 0.2 to' 0.8$ of Mo when desired, the balance being substantially ?e. ej) ^ O /> DETAILED DESCRIPTION OF THE INVENTION.

In the description to follow, the percentages are all by weight.

The heat resistant cast steel of the present 5 invention contains the following components in the following proportions in terms of $ by weight: C 0.3 - 0.6, 0 < Si ^ 2.0 , 0 < Mn ^ 2.0, Cr 20 - 30, Ni 30-40, Nb + Ta 0.3 - 1.5, W 0.5-3-0, N 0.04-0.15 and ° B 0.0002 - 0.004, the steel also containing Ti and Al in the combination of Ti 0.04 - 0.15 and 'Al 0.02-0.07, or rTi 0.04 — 0.50 and Ul 0.07 - 0.50, the steel further containing v/hen desired: Mo 0.2-0.8, the balance being substantially Fe .

The components of the cast steel of the invention and the proportions of the components will be described below in detail.

C imparts good castability to cast steel, forms 5 primary carbide in the presence of the Nb to be described later and is essential in giving enhanced creep fracture strength. At least about 0.3°h of C is therefore required. With the increase of the amount of C, the creep fracture strength increases, but if an excess of G is present, an 10 excess of secondary carbide will precipitate, resulting in greatly reduced toughness and impaired weldability.

Thus the amount of G should not exceed about 0.6$.

Si serves as a deoxiaant during melting of the components and is effective for affording improved anti-15 carburizing properties. However, the Si content must be up to about 2.0$ or lower since an excess of Si will lead to impaired weldability.

Mn functions also as a aeoxidant like Si, while S in molten steel is effectively fixed'and rendered 20 harmless by Mn, but a large amount ox Mn, if present, renders the steel less resistant to oxidation. The upper limit of Mn content is therefore about 2.0$. 2 or- ^ /s i ' V In the presence of Ni, Cr forms an austenitic cast steel structure, giving the steel improved strength at high temperatures ana increased resistance to oxidation. These effects increase with increasing Gr content. At 5 least about 20$ of Cr is used to obtain a steel having sufficient strength and sufficient resistance to oxidation especially at high temperatures of at least about 1000° C. However, since the presence of an excess of Cr results in greatly reduced toughness after use, the upper limit of "10 the Cr content is about 3C$ .

■ As described above, Ni, when present conjointly with Cr, forms an austenitic cast steel of stabilized structure, giving the steel improved resistance to oxidation and enhanced strength at high temperatures. To 15 make the steel satisfactory in oxidation resistance and strength especially at high temperatures of at least about 1000° C, at least about 30p of Ni must be used. Although these two properties improve with the increase of the Ni content, the effects level off when the Ni content exceeds 20 about 40$, hence economically unfavorable, so that the upper limit of the Ni content is about 40j£. 2012 Nb is effective in improving creep fracture strength and anti-carburizing properties, provided that at least about O.y/o of Nb is used. On the other hand, when containing an excess of Nb, the steel will have 5 decreased creep fracture strength. The upper limit of the Nb content is therefore about 1.5°'. Usually Nb inevitably contains Ta which has the same effect as Nb. When Nb contains Ta, accordingly, the combined amount of Nb and Ta may be about 0.3 to 1.5$.

When in combination with Nb, W contributes to the improvement of strength at high temperatures. At least about 0.5$ of W is used for this purpose, but the upper limit of the W content is about 3.0$ since use of larger amounts of W leads to reduced resistance to oxida-15 tion.

The steel of this invention has the greatest feature in that it contains specified amounts of N, Ti, Al and B, in addition to the foregoing elements. When desired, the steel further contains Mo. These elements, 20 when used conjointly, produce remarkably improved characteristics at high temperatures. This effect is not achievable if any one of N, Ti, Al and B is absent.

N serves in the form.of a solid solution to stabilize and reinforce the austenitic phase, forms a 25 nitride and carbonitride with Ti, etc., produces refined 201260 grains v/hen finely dispersed in the presence of Al and B and prevents grain growth, thus contributing to the improvement of high-temperature strength and resistance to thermal shock. It is desired that the N content be 5 at least about 0.04$ to achieve these effects sufficiently. Preferably the upper limit of the N content is about 0.15$ since the presence of an excess of N permits excessive precipitation of nitride and carbonitride, formation of coarse particles of nitride and carbonitride and impair-10 ment of resistance to thermal shock.

When combining with C and N in steel, Ti forms a carbide, nitride and carbonitride, thereby affording improved high-temperature strength and enhanced resistance to thermal shock. Especially Ti acts synergistically with 15 Al, producing enhanced anti-carburizing properties. It is preferable to use at least about 0.04$ of Ti to assure these effects. While improvements are achieved in creep fracture strength, resistance to thermal shock ana anti-carburizing properties with the increase of the Ti content, 20 use of a large amount of Ti results in coarse particles of precipitates, an increased amount of oxide inclusions and somewhat reduced strength. Accordingly, when high strength is essential, the upper limit of the Ti content is preferably about 0.15$. Further when the Ti content 25 exceeds about 0.5$, greatly reduced strength will result, /' /C f* so that the Ti content should not exceed about 0.5i- even if resistance to carburizing is critical.

Al affords improved creep fracture strength and, when present conjointly with Ti, achieves a remarkable 5 improvement.in resistance to carburizing. Preferably at least about 0.02$ of Al should be used to give improved creep fracture strength. Although higher strength at high temperatures and higher resistance to carburizing will result with increasing Al content, use of an excess 10 of Al conversely leads to reduced strength. Accordingly v/hen strength at high temperatures is essential, the upper limit of the Al content is preferably about 0.07$.

However, when it is desired to obtain a steel which is comparable to conventional H? materials in high-tempera-15 ture strength but ha.s improved anti-carburizing properties, amounts at least larger than about 0.07$ are desirable. Nevertheless extremely decreased strength will result if the Al content exceeds about 0.5$. Accordingly the Al content should not be higher than about 0.5$. 20 B serves to form reinforced grain boundaries in the matrix of steel, prevents formation of coarse particles of Ti precipitates but permits precipitation of fine particles thereof and retards agglomeration of particles of precipitates, thereby affording improved creep fracture strength. 25 For this purpose it is desirable to use at least about 0.0002f'j of B. On the other hand, use of a large amount of B does not result in a corresponding increase in strength and entails reduced weldability. Preferably, therefore, the upper limit of the B content is about 0.004$.

Mo, which is used when required, contributes to the improvement in high-temperature strength if used in combination with Nb and V7. To produce this effect, L-io is used in an amount of at least about 0.2$. However, if a large excess of to is present, lower resistance to oxidation will result, so that Mo, when used, is used in an amount of up to about 0.8$.

Impurities, such as P and S, may be present in amounts which are usually allowable for steels of the type described.

The high-temperature characteristics of the cast steel of this invention will.be described below in detail with reference to examples.

Cast steels of various compositions were prepared in an induction- melting furnace (in the atmosphere) and made into ingots (136 mm in outside diameter, 20 mm in wall thickness and 500 mm in length) by centrifugal casting. Tables 1, 3, 5 and 7 show the chemical compositions of the steel specimens thus obtained. *7) ^ ^ ^3 U U /? I ^ Test pieces were prepared from the steel specimens and tested for creep fracture strength, resistance to thermal shock and resistance to carburizing by the following methods.

Test 1: Creep fracture test According to JIS Z 2272 under the following two conditions: (A) Temperature 1093°C, Load 1.9 kgf/mm2 (B) Temperature 850°C, Load 7.3 kgf/mm^ Test 2: Thermal shock resistance test A test piece used was made in the form of a disc 30 mm x 8 mm thickness) having a hole (0 20 mm) opened therethrough at its center point in the position of 17 nun inside from the peripheral face.

The procedure of heating the test piece at 900°C for 30 minutes and thereafter cooling the test piece with water at temperature of-about 25°C was repeated. Every time this procedure was repeated 10 times, the length of the crack occurring in the test piece was measured. The resistance to thermal shock was expressed in terms of the number of repetitions when the length of the crack reached 5 mm. 2012.60 Test 3: Carburizing resistance test A test piece used was made in the cylindrical form (0 12 x 60 mm length).

After holding.the test piece in a solid carburizer (Durferrit carburizing granulate KG 30, containing BaCO^) at a temperature of 1100°C for 300 hours, a 1-mm-thick surface layer (hereinafter referred to as "layer 1") was removed from the test piece by grinding to obtain particles. The resulting surface of the test piece was further ground to remove another 1-mm-thick layer (to a depth of 2 mn from the original surface, hereinafter referred to as "layer 2") to obtain particles. The particles of each layer v/ere analyzed to determine the C content„ The resistance to carburizing is expressed in terms of the increment (%) of the C content.

The carburizing resistance test was conducted only for the steel specimens shown in Tables 3 and 7 • The results of the foregoing tests are listed in Table 2, if, 6 or 8, and will be described in the following examples: Examle 1 Of the steel specimens listed in Table 1, Specimens No.l to No.6 are according to the invention 2 0 1 2( and contain about 0.0^ to 0.15% of Ti and about 0.02 to 0.07% of Al but are free from Mo. Specimens No.7 to No.22 are comparison steels, of which Specimen No.7 is a HP material containing Nb and W, Specimens No.8 to No.l^f are 5 free from at least one of Ti, Al and B, and Specimens No.15 to No.22 contain N, Ti, Al and B in amounts outside the foregoing ranges specified by the invention.

Table 2 shows the results of the creep fracture test and thermal shock resistance test. Specimens No. 1 10 to No. 6 have exceedingly higher creep fracture strength at high temperatures than Specimen No. 7, i.e. Nb- and Vi'-containing PIP material which is considered to be excellent in such strength ana the other comparison steels. The comparison steels which, are free from at 15 least one of N, Ti, Al and B or contain these elements in excessive or insufficient amounts are inferior in creep fracture strength. This indicates that the outstanding characteristics can be obtained- only v/hen these elements are conjointly present in amounts v/ithin the 20 specified ranges. It is especially noteworthy that the steels of this invention exhibit much higher creep fracture characteristics at high temperatures above 1000° G, e.g. at 1093° C, than at temperatures below 1000° C, e.g. at 850° C. 201260 It is also noted that the steels of the invention have much higher resistance to thermal shock than the HP material containing Nb and W and the other comparison steels. The remarkable resistance is of course attributable 5 to the conjoint use of N, Ti, Al and B.

Spec No. -p-i Table 1 Chemical compositions of steel specimens (wt. %) Si Mn Cr Ni Nb+Ta W N, Ti Al B Remarks 1 0. 46 1. 21 0. 63 . 82 . 02 1 .27 1 .13 0. 09 0 .05 0 ' CO o 0 .0010 With N, Ti, contents Al, B 2 0. 34 1. 16 0. 76 23. 41 33. 48 0 .47 0 .71 0. 06 0 .05 0 .07 0 .0025 II d o 3 0. 56 1. 0. 85 28. 60 39. 1 .36 2 .45 0. 07 0 .06 0 .07 0 .0029 II •H -P ti 4 0. 45 1. 28 0. 72 . 90 . 08 1 .28 1 .09 0. 08 0 .07 0 .04 0 .0021 II 0) > ci 0. 43 1. 24 0. 70 26. 89 34. 67 1 .15 1 .08 0. 0 .10 0 .07 0 .0032 II 0) 6 0. 45 1. 0. 65 26. 78 . 16 1 .24 1 .10 0. 13 0 .09 0 .07 0 .0025 EH 7 0. 44 1. 27 0. 65 26. 01 . 40 1 .21 1 .05 — — — HP mat. with W contents Nb, 8 0. 43 1. 23 0. 76 26. 52 . 11 1 .17 1 .11 0. 08 - - - Ti-, Al-, B- free 9 0. 43 1. 0. 73 . 74 . 17 1 .15 1 .15 0. 08 0 .04 - - A1-, B-free ti 0. 44 1. 0. 62 . 70 . 32 1 .27 1 .02 0. 09 0 .13 - - II o CO •H £. 11 0. 42 1. 19 0. 78 26. 11 . 37 1 .22 0 .99 0. - 0 .03 - Ti-, B-free t3 ft a 12 0. 43 1. 17 0. 76 26. 27 . 07 1 .14 1 .06 0. - 0 .07 - If o o 13 0. 43 1. 24 0. 70 26. 51 . 19 1 .14 1 .06 0. 09 0 .06 0 .03 - B-free 14 0. 45 1. 26 0. 61 26. 07 . 21 1 .24 1 .10 0. 08 0 .10 0 .06 II N> O is) o Spe No . 16 17 18 19 21 22 Table 1 (continued) Si Mn Cr Ni Nb+Ta W N Ti A,1 B Remarks 0 .45 1 .26 o • —j o 26 .21 .07 1 .20 1 .11 o o 0 .03 0 .05 0.0016 Ti deficient 0 .45 1 .17 0.66 26 .17 .12 1 .27 1 .02 0.10 0 •19 0 .06 0.0012 Ti excessive 0 .43 1 .22 0.68 26 .27 34 .92 1 .27 1 .07 0.08 0 .08 0 .01 0.0010 Al deficient 0 .44 1 .27 0.67 26 .20 34 .87 1 .19 1 .14 0.00 0 .07 0 .11 0.0012 Al excessive 0 .43 1 • 19 0.67 26 .19 .10 1 .15 1 .12 0.10 0 .07 0 .05 0.0001 B deficient 0 .43 1 .18 0.69 26 .15 .02 1 .26 1 .10 0.10 0 .08 0 .05 0.0049 B excessive 0 .44 1 .17 0.67 26 .25 .21 1 .26 1 .11 0.03 0 .09 0 .06 0.0015 N deficient 0 .44 1 .25 0.72 26 .09 .11 1 .18 1 .08 0.18 0 .09 0 .06 0.0021 N excessive Pl 0 in *H 01 ft a o o 2.0 1260 Sxiec No . 1 2 3 k 6 7 8 9 11 12 13 14 16 17 18 19 21 22 Table 2 ' Test results Creep fracture strength (kgi/mm^) - Condition (.A J Condition (B) Resistance to Remarks thermal shock (times) 202 219 ZkO 221 250 246 80 91 113 127 115 131 133 143 88 127 92 119 103 125 92 154 156 160 170 167 179 172 73 83 105 116 104 114 110 122 83 105 84 100 77 113 79 137 320 3k0 320 350 360 150 140 190 210 170 190 240 280 Invention Comparison It II 210 130 -.16- D12 60 Examnle 2 Of the steel specimens shown in Table 3 Specimens No. 23 "to No. 28 are according to the invention and contain Ti, Al and Mo within the ranges of about 0.04 5 to 0.15$ Ti, about 0.02 to 0.07$ Al and about 0.2 to 0.8$ Mo. Of Specimens No. 29 to No. 44 prepared for corn'oar-ison, Specimen No. 29 is a HP material containing Nb, W and Ivio, Specimens No. 30 to No .36 are free from at least one of Ti, Al and B, and Specimens No. 37 to No . 44 10 contain N, Ti, Al and B in amounts outside the ranges specified in this invention. the steels of the invention have exceedingly higher creep fracture characteristics and resistance to thermal shock than the HP material containing Nb, W and Mo ana the other comparison steels due to the conjoint presence of N, Ti, A1 and B. .

Table 4 shows the results of creep fracture test and thermal shock resistance test.

Table.4 reveals that as is the case with Example I, pec o. 23 24 26 27 28 29 31 32 33 34 36 Table 3 Chemical compositions of steel specimens (wt. %) C Si Mn Cr Ni Nb+Ta W Mo N Ti Al B Remarks With N, Ti, Al, 0.44 1.20 0.64 25.17 36.20 1.28 1.02 0.48 0.11 0.04 0.03 0.0008 B contents 0.37 1.05 0.68 22.75 33.05 0.45 0.66 0.35 0.06 0.07 0.05 0.0024 " 0.51 1.09 0.83 29.01 38.68 1.29 2.11 0.73 0.05 0.08 0.05 0.0027 " 0.43 1.23 0.69 25.98 35.76 1.23 1.09 0.42 0.09 0.07 0.05 0.0019 " 0.45 1.23 0.77 25.73 35.19 1.19 1.13 0.43 0.08 0.12 0.07 0.0032 0.44 1.21 0.75 26.02 35.08 1.15 1.10 0.41 0.14 0.08 0.07 0.0025 " 0. 42 1 ro O 0. 71 26. 12 .37 1 .29 1 .10 0. 42 „ _ HP w, mat. with Nb, Mo contents 0. 43 1 .17 0. 72 26. 24 .82 1 .11 1 .07 0. 39 0.09 - - - Ti Al-, B-free 0. 43 1 .26 0. 73 . 97 36 .07 1 .27 1 .05 0. 37 0.08 0 .05 - - Al B-free 0. 45 1 .31 0. 68 . 81 .51 1 .25 0 .97 0. 46 0.09 0 .12 - - II 0. 44 1 .28 0. 65 26. 37 .11 1 .20 1 .11 0. 45 0.07 - 0 .02 - Ti -, B-free 0. 44 1 .32 0. 65 26. 46 .55 1 .20 1 .07 0. 32 0.08 - 0 .06 - II 0. 45 1 .26 0. 71 26. 36 •12 1 .19 1 .06 0. 40 O • o 0 .05 0 .03 - B- free 0. 46 1 .21 0. 73 26. 33 36 .23 1 .28 1 .06 0. 41 0.08 0 .09 0 .07 _ \\ Spec. No .

Table 3 (continued) Si Mn Cr Ni Nb+Ta W Mo N Ti Al B Remarks: 37 0.44 1.21 0.75 26.07 36.21 1.17 1.08 0.43 0.09 0.02 0.06 0.0015 Ti deficient 38 0.44 1.25 0.77 26.12 35.92 1.19 1.11 0.41 0.08 0.20 0.07 0.0017 Ti excessive 39 0.45 1.31 0.67 26.15 35.87 1.24 1.06 0.39 0.09 0.08 0.01 0.0018 Al deficient / 40 0.43 1.28 0.65 25-95 36.07 1.25 1.06 0.39 0.10 0.09 0.12 0.0021 Al excessive 41 0.43 1.22 0.69 25.89 35.23 1.20 1.13 0.42 0.11 0.09 0.05 0.0001 B deficient 42 0.45 1.22 0.70 26.34 35.35 1.15 1.17 0.42 0.10 0.07 0.07 0.0055 B excessive ( 43 0.44 1.30 0.72 26.27 35.18 1.21 1.10 0.45 0.02 0.09 0.06 0.0016 N deficient f ^ 0.45 1.25 0.67 26.19 35.08 1.24 1.11 0.41 0.19 0.10 0.07 0.0022 N excessive o w •H f-l aS ft a o o esy, 0 1 2 6 Table 4 Test results Spec. Creep fracture strength.

No . (kgi /mm^ ) Condition (A) Condition (3) Resistance to thermal shock (times) Remarks 23 213 164 310 24 225 168 350 260 180 330 26 233 176 350 27 264 189 380 28 259 181 - 29 85 77 160 ' 96 87 130 31 120 111 200 32 134 123 230 33 122 110 180 34 138 121 210 141 116 250 36 151 129 280 37 88 87 - 38 125 111 - 39 92 88 - 40 131 105 - 41 95 82 - 42 138 120 - 43 97 84 240 44 162 144 140 Invention Comparison tt Tf II tl 201260 Examnle 3 Of the steel specimens shewn in Table 5, Specimens No. 45 to No. 50 are according to the invention. These specimens contain Ti and Al within the ranges of 5 about 0.04 to 0.50$ Ti and about 0.07 to 0.50$ Al but are free from Mo. Of Specimens No. 51 "to No. 55 prepared for comparison, Specimen No. 51 is a HP material containing Nb and W (but free from any of N, Ti, Al and B) , and Specimens No. 52 to No. 55 contain N, Ti, Al and B in 10 amounts outside the foregoing ranges specified by this invention.

Table 6 shows the results of creep fracture test, thermal shock resistance test and carburizing resistance tes The steels of the invention prepared in this 15 example are lower than those in Examples 1 and 2 in creep fracture strength and thermal shock resistance because v they have higher Ti and Al contents but, nevertheless, they are much superior in high-temperature creep fracture strength and resistance to thermal shock, to the Nb- and 20 V/-containing HP material, i.e. Specimen 51 , which is considered to be higher in high-temperature creep fracture strength than other conventional steels, "the steels of the invention further similarly superior to the other comparison steels.

The carburizing resistance listed in Table 6 is 2 01260 expressed in terms of weight percent increment of C content. Thus the smaller the value, the smaller is the increment and the higher is the resistance to carburizing.

Table 6 reveals that Ti and Al act sj^nergistically 5 to give the steels of the invention sufficient creep fracture strength and thermal shock resistance and outstanding resistance to carburizing.

Spe No . ,45 46 47 48 49 50 51 52 53 54 55 Table 5 Ghemical compositions of steel specimens (wt. $) G Si Mn Cr Ni Nb+Ta W N Ti Al B Remarlcs 0.45 1 .21 0.70 .72 .06 1.15 1.10 0.08 0 .20 0.15 0.0023 The invention 0.34 1 .19 0.70 23.26 33.50 0.38 0.70 0.07 0 .16 0.18 0.0022 it 0.55 1 .15 0.72 28.34 39.25 1.18 2.33 0.05 0 • 16 0.17 0.0024 it 0.44 1 .19 0.66 .63 .12 1.20 1.07 0.07 0 .17 0.19 0.0020 II 0.44 1 .27 0.67 26.20 / 34.87 1.19 1.14 0.08 0 .07 0.11 0.0012 II 0.45 1 .20 0.71 .77 .18 1.25 1.17 0.08 0 .08 0.12 0.0017 II 0.44 1 .27 0.65 26.01 .40 1.21 1.05 - - - Comparison 0.43 1 .28 0.72 26.07 .15 1.11 1.15 0.07 0 .02 0.12 0.0015 II 0.44 1 .12 0.70 26.08 34.62 1.27 1.10 0.07 0 .56 0.11 0.0018 II 0.45 •1 .10 0.75 26.01 .17 1.20 1.08 0.08 0 .17 0.01 0.0011 II 0.44 1 .13 0.79 .68 .11 1.15 1.16 0.09 0 .19 0.53 0.0014 II • • • • • • Table 6 Test results i ro -p- i Spec. No. 45 46 47 48 49 50 51 52 53 54 55 Creep fracture strength (Icgf/mm^) Condition (A) Condition (B) Resistance to Resistance to carburizing Remarks 111 108 125 114 119 129 80 95 64 100 58 91 90 115 96 100 114 73 82 57 83 54 thermal shock (times) 170 180 170 180 180 150 150 110 140 100 zing (G content increment, $) Layer 1 Layer 2 0.85 0.91 0.84 0.87 1.00 1.02 1.61 1.23 1.04 1.29 1.03 0.44 0.52 0.50 0.47 0.51 0.54 0.92 0.66, 0.56 0.74 0.57 Invention Comparison N) O ro 2 012 6 0 Example 4 Of tiie steel specimens shown in Table 7, Specimens No. 56 to No. 61 are according to the invention and contain Ti, Al ana Mo within the ranges of about 0.04 to 0.50$ Ti, about 0.07 to 0.50$ Al and about 0.2 to 0.8$ Mo. Of Specimens No. 62 to No. 66 prepared for comparison. Specimen No. 62 is a HP material containing Nb, Mo and vV (but free from any of N, Ti, Al and 3), and Specimens No. 63 to No. 66 contain N, Ti, Al and B, the content of Ti or Al being outside the range specified by the invention.

Table 8 shows the results of creep fracture test, thermal shock resistance test and carburizing resistance test.

For the same reason given in Example 3, the steels of this invention prepared in this example are lower than those in Examples 1 and 2 in respect of creep fracture strength and thermal shock resistance, but are much superior in high-temperature creep fracture strength and thermal shock resistance to the Nb-, W- and Eo-containing HP material, i.e. Specimen 63, which is considered to be higher than other conventional steels in high-temperature creep fracture strength and also to the other comparison steels.

Due to the synergistic effect of Ti ana Al, the * steels of the invention have higher carburizing resistance than the comparison steels.

Spec Mo . 56 '57 58 59 60 61 62 63 64 65 66 Table 7 Ghemical compositions of steel specimens (wt. <>/<,) c Si Mn Cr Ni Nb+Ta W • Mo N Ti Al B Remarks 0.45 1.27 0.73 25-71 -82 1.10 1.12 0.45 0.08 0.18 0.15 0 .0018 The invention 0.33 1.16 0.70 24.10 33.16 0.44 0.62 0.34 0.08 0.18 0.19 0 .0027 tt 0.56 1.11 0.68 28.35 38.78 1.40 2.26 0.75 0.07 0.17 a. 20 0 .0025 tt 0.44 1.22 0.69 25-63 -24 1.21 1.10 0.40 0.07 0.17 0.17 0 .0022 II 0.43 1.28 0.65 25-95 36.07 1.25 1.06 0.39 0.10 0.09 0.12 0 .0021 II 0.45 1.20 0.75 25.77 .26 1.27 1.02 0.41 0.09 0.07 0.14 0 .0017 II 0.42 1.20 0.71' 26.12 -37 1.29 1.10 0.42 - - - - ■ Comparison 0.43 1.27 0.77 26.15 .09 1.17 1.16 0.45 0.08 0.02 0.12 0 .0011 It 0.44 1.12 0.75 26.13 34-91 1.25 1.14 0.37 0.09 0.56 0.10 0 .0017 II 0.45 1.15 0.70 26.11 -21 1.21 1.27 0.40 0.10 0.17 0.01 0 .0012 II 0.44 1.10 0.67 25 -78 -20 1-15 1.10 0.45 0.10 0.19 0.54 0 .0027 II Spec No . 56 57 58 59 6o 61 62 63 64 65 66 Table 8 Test results Creep fracture strength Resistance to Resistance to carburizing (kfff/mm^) thermal shock (C content increment, jc) Condition (A) Condition (B) (times) Layer 1 Layer 2 117 96 180 0.81 0.42 115 95 180 0.81 0.45 120 105 180 0.80 0.47 121 102 180 0.83 0.45 131 105 - 0.95 0.48 136 121 190 0.97 0.51 85 77 160 1.53 0.87 101 86 160 1.17 0.63- 67 60 110 0.99 0.53 105 87 150 1.23 0.70 61 57 110 0.98 0.54

Claims (7)

2012 60 The heat resistant cast steel of this invention is thus exceedingly superior to the conventional HP materials in respect of high-temperature creep fracture strength and resistance to thermal shock. Especially when high resistance to carburizing is required of the steel, the steel can be improved in this property while minimising the reduction of the high-temperature creep fracture strength and thermal shock resistance "by incorporating Ti and Al into the steel in amounts within the ranges specified by the invention. Accordingly the present steel is well suited as a material for various apparatus and parts for use at temperatures above 1000° C, for example, for ethylene cracking tubes and reforming tubes in the petrochemical industry or for hearth rolls and radiant tubes in iron and steel and related industries. The scope of the invention is not limited to the foregoing description, but various modifications can be made with ease by one skilled in the art without departing from the spirit of the invention. Such modifications are therefore included within the scope of the invention. -28- 2 0126 i WHAT^WE CUIM IS; flh. at—Ls—claimed—i-s-:

1. A heat -resistant cast steel containing the following components in the following proportions in terms of <fc by weight: C 0.3 - 0.6, 0 ^ Si ^ »v o • CM 0 < Mn 4 2.0, Cr 20 - 30, Ni 0 1 O Nb + Ta 0.3 - 1.5, W 0.5 - 3.0, N 0.04 - 0.15, B 0.0002 - 0.004, Ti 0.04 - 0.50 and Al 0.02 - 0.50, 15 the balance being substantially Fe.

2. A heat resistant cast steel as defined in claim 1 wherein 0,04 to 0.15% by weight of Ti and 0.02 to 0.07% by weight of Al are contained.

3. A heat resistant cast steel as defined in claim 1 wherein 0.04 to 0.50% by weight of Ti and 0.07 to 0.50% by weight of Al are contained.

4. A heat resistant cast steel containing the following components in the following proportions in terms of "Jo by weight: -29- 2 012 6 0 c 0.3 - 0.6, 0 < Si ^ 2.0, 0 < Mn O • CM Cr 20 - 30, Ni 30-40, Nb 4 - Ta 0.3 - 1.5, W 0.5 - 3.0, N 0.04 - 0.15, B 0.0002 - 0.004, Ti 0.04 - 0.50, Al 0.02 - 0.50 and Mo 0.2 - 0.8, the balance being substantially Fe.

5. A heat resistant cast steel as defined in claim 4 wherein 0.04 to 0.15% by weight of Ti and 0.02 to 0.07% by weight of Al are contained.

6. A heat resistant cast steel as defined in claim 4 wherein 0.04 to 0.50% by weight of Ti and 0.07 to 0.50% by weight of Al -are contained.

7. A heat resistant cast.steel as claimed in any one of the preceding'claims substantially as hereinbefore described with reference to any example thereof. -30- „ - - "•* By Hie/Their authorised Agents, A. J. & S0N Per