Heat Resistant Cast Iron-Nickel-Chromium Alloy

The present invention relates to heat resistant cast iron-nickel-chromium alloy, and more particularly to austenitic heat resistant cast iron-nickel-chromium alloy having the composition of Cr, Ni, and Nb which is excellent in creep fracture strength at high temperatures and in resistance to thermal impact or carburizing, with further use of the composition of N, Ti, Al and B, especially under the severe operating conditions at temperature above 1000.degree. C.

HK 40 which is a heat resistant cast alloy containing Ni and Cr (25Cr-20Ni steel, see ASTM A 608) and HP materials (25Cr-35Ni steel, see ASTM A 297) have been used as materials for ethylene cracking tubes in the petrochemical industries. With the elevation of operating temperatures in recent years, it has been required to improve the high-temperature characteristics of such materials. To meet this requirement, HP materials containing Nb have been developed and placed into use. However, with the recent tendency toward severer operating conditions, it is desired to provide materials which are superior to such HP materials containing Nb in respect of high-temperature creep fracture strength and resistance to thermal shock or carburizing.

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 resistant cast iron-nickel-chromium alloy containing Cr, Ni and Nb as the essential components and found that the alloy can be remarkably improved in high-temperature creep fracture strength and resistance to thermal shock and to carburizing especially under temperature range above 1000.degree. C., by containing N, B, Ti and Al therein. Thus this invention has been accomplished.

Stated specifically, the present invention provides a heat resistant cast iron-nickel-chromium alloy containing about 0.3 to 0.6% (by weight, the same as hereinafter) of C, up to 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 0.04 to 0.15% of N, about 0.0002 to 0.004% of B, about 0.04 to 0.50% of Ti and about 0.07 to 0.50% of Al, the balance being substantially Fe.

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

The heat resistant cast iron-nickel-chromium alloy of the present invention contains the following components in the following proportions in terms of % by weight:

C: 0.3-0.6

0<Si.ltoreq.2.0

0<Mn.ltoreq.2.0

Cr: 20-30

Ni: 30-40

Nb+Ta: 0.3-1.5

N: 0.04-0.15

B: 0.0002-0.004

Ti: 0.04-0.50 and

0.07<Al.ltoreq.0.50

the balance being substantially Fe.

In the course of the research which has matured to the present invention, we have also found a heat resistant cast alloy containing the following components in the following proportions in terms of % by weight:

C: 0.3-0.6,

0<Si.ltoreq.2.0,

0<Mn.ltoreq.2.0,

Cr: 20-30

Ni: 30-40

Nb+Ta: 0.3-1.5,

N: 0.04-0.15,

Ti: 0.04-0.50, 0.07<Al.ltoreq.0.50

B: 0.0002-0.004,

W: 0.5-3.0,

Mo: 0.2-0.8 and

Fe: balance.

This heat resistant cast alloy, as containing W and Mo unlike the cast iron-nickel-chromium alloy of the invention, has higher resistance to thermal impact than the iron-nickel-chromium alloy of the invention.

In respect to creep fracture strength at high temperatures, the alloy is superior to the conventional HP materials but is slightly inferior to the cast iron-nickel-chromium alloy of the invention. Under conditions in which satisfactory creep fracture strength at high temperatures is required in addition to economical reasons, the cast iron-nickel-chromium alloy of this invention is preferable to use even though the said alloy is generally superior to the cast iron-nickel-chromium alloy of this invention in both creep fracture strength and resistance to thermal shock.

The components of the cast iron-nickel-chromium alloy of the invention and the proportions of the components will be described below in detail.

C imparts good castability to cast steel, or alloy forms 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% of C is therefore required. With the increase of the amount of C, the creep fracture strength increases, but if an excess of C is present, an excess of secondary carbide will precipitate, resulting in greatly reduced toughness and impaired weldability. Thus the amount of C should not exceed about 0.6%.

Si serves as a deoxidant during melting of the components and is effective for affording improved anticarburizing 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 deoxidant like Si, while S in molten or alloy is effectively fixed and rendered harmless by Mn, but a large amount of Mn, if present, renders the or alloy less resistant to oxidation. The upper limit of Mn content is therefore about 2.0%.

In the presence of Ni, Cr forms an austenitic cast alloy structure, giving the steel improved strength at high temperatures and increased resistance to oxidation. These effects increase with increasing Cr content. At least about 20% of Cr is used to obtain a alloy having sufficient strength and sufficient resistance to oxidation especially at high temperatures of at least about 1000.degree. C. However, since the presence of an excess of Cr results in greatly reduced toughness after use, the upper limit of the Cr content is about 30%.

As described above, Ni, when present conjointly with Cr, forms an austenitic cast alloy of stabilized structure, giving the alloy improved resistance to oxidation and enhanced strength at high temperatures. To make the alloy satisfactory in oxidation resistance and strength especially at high temperatures of at least about 1000.degree. C., at least about 30% 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 about 40%, hence economically unfavorable, so that the upper limit of the Ni content is about 40%.

Nb is effective in improving creep fracture strength and anti-carburizing properties, provided that at least about 0.3% of Nb is used. On the other hand, when containing an excess of Nb, the alloy will have 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%.

The alloy 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. These elements, when used conjointly, produce remarkably improved characteristics at high temperatures. Especially, under the use at high temperature above 1000.degree. C., the alloy of the invention provides excellent features in creep fracture strength, resistance to thermal shock and to carburizing. This effect is not achievable if any one of N, Ti, Al and B is absent.

That is to say, Ti forms compounds such as carbide, nitride and carbonitride in combination with C and N. B and Al finely disperse and precipitate the said compounds to reinforce grain boundaries and to enhance resistance to cracking on the grain boundaries. Remarkable improvement in high temperature strength, that is, creep fracture strength and in high temperature characteristics of resistance to thermal shock is thus obtained. Furthermore, Ti contributes to remarkable improvement in anti-carburizing property owing to synergistic effect with Al.

N serves in the form of a solid solution to stabilize and reinforce the austenitic phase, forms nitride and carbonitride with Ti, etc., produces refined grains when 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 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 impairment of resistance to thermal shock.

As stated above, when combining with C and N in steel, Ti forms carbide, nitride and carbonitride, thereby affording improved high-temperature strength and enhanced resistance to thermal shock. Especially Ti acts synergistically with 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 and anti-carburizing properties with the increase of the Ti content, 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 exceeds about 0.5%, greatly reduced strength will result, so that the Ti content should not exceed about 0.5% even if resistance to carburizing is critical.

Al affords improved creep fracture strength and, when present conjointly with Ti, achieves a remarkable 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 high resistance to carburizing will result with increasing Al content, use of an excess of Al conversely leads to reduced strength. Accordingly when 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 an alloy which is comparable to conventional HP materials in high-temperature strength but has improved anti-carburizing properties, amount 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%. Presence of a layer rich in Al can be detected by Electron Probe Micro Analyzer on the surface layer portion of the Ti and Al containing steel or alloy specimen for which carburizing treatment was applied. The layer rich in Al serves to a notable effect of preventing carburization.

B serves to form reinforced grain boundaries in the matrix of a steel or alloy, 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. For this purpose it is desirable to use at least about 0.0002% 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%.

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

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

Cast alloys 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. Table 1 shows the chemical compositions of the alloy specimens thus obtained.

Of the alloy specimens listed in Table 1, Specimens No. 1 to No. 4 are according to the invention. Specimens No. 5 to No. 9 are comparison alloys, of which Specimen No. 5 is a HP material containing Nb (free from any one of N, Ti, Al and B), and Specimens No. 6 to No. 9 contain N, Ti, Al and B, the content of Ti or Al being outside the range specified by the invention.

Test pieces were prepared from the alloy specimens and tested for creep fracture strength, resistance to thermal shock and resistance to carburizing by the following methods.

According to JIS Z 2272 under the following two conditions:

(A) Temperature 1093.degree. C., load 1.9 kgf/mm.sup.2

(B) Temperature 850.degree. C., load 7.3 kgf/mm.sup.2

A test piece used was made in the form of a disc (.0.50 mm.times.8 mm thickness) having a hole (.0.20 mm) opened therethrough at its center point in the position of 17 mm inside from the peripheral face.

The procedure of heating the test piece at 900.degree. C. for 30 minutes and thereafter cooling the test piece with water at temperature of about 25.degree. C. was prepared. 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.

A test piece used was made in the cylindrical form (.0.12.times.60 mm in length).

After holding the test piece in a solid carburizer (Durferrit carburizing granulate KG 30, containing BaCO.sub.3) at a temperature of 1100.degree. C. for 300 hours, 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 mm from the original surface, hereinafter referred to as "layer 2") to obtain particles. The particles of each layer were analyzed to determine the C content. The resistance to carburizing is expressed in terms of the increment (%) of the C content. Thus the smaller the value, the smaller is the increment and the higher is the resistance to carburizing.

The results of the foregoing three kinds of tests are listed in Table 2.

As shown in Table 2, the alloy of this invention has exceedingly higher creep fracture strength at high temperatures than specimen No. 5, i.e. Nb-containing conventional material which is considered to be excellent in such strength and the other comparison alloy. It is to be noted that high strength of creep fracture is maintained especially even at temperature range above 1000.degree. C. The present alloy is also superior to the conventional alloy and other comparison steel in respect of resistance to thermal shock.

In the test of resistance to carburizing, the carbon increment is smaller than the conventional alloy (specimen No. 5) by half or less, and is extremely small in comparison with other comparison alloys (specimens No. 6 to No. 9). This is due to synergistic effect of Ti and Al.

The heat resistant cast iron-nickel-chromium alloy of this invention is thus exceedingly superior to the conventional Nb containing HP materials or the like in respect to high-temperature creep fracture strength and resistance to thermal shock.

Accordingly the present iron-nickel-chromium alloy is well suited as a material for various apparatus and parts for use at temperature above 1000.degree. C., for example, for ethylene cracking tubes and reforming tubes in the petrochemical industry or for hearth rolls and radiant tubes in iron 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.