FORM 2 THE PATENTS ACT 1970 (39 of 1970) & The Patents Rules, 2003 COMPLETE SPECIFICATION (See section 10 and rule 13) 1. HEAT-RESISTANT ALLOY, AND REACTION TUBE 2. 1. (A) KUBOTA CORPORATION (B) Japan (C) 2-47, Shikitsuhigashi 1-chome, Naniwa-ku, Osaka-shi, Osaka 5568601 Japan The following specification particularly describes the invention and the manner in which it is to be performed. [Technical Field] [0001] The present invention relates to a heat-resistant alloy used for, for example, a reaction tube for producing a hydrocarbon gas, and more specifically to a heat-resistant alloy which can suitably form an Al oxide layer on the surface. [Background Art] [0002] Olefin hydrocarbons such as ethylene and propylene and styrene hydrocarbons such as styrene monomers are produced by flowing a hydrocarbon raw material gas and vapor fluid in a reaction tube which is heated from the outside and thermally decomposing the raw material fluids by heating to the reaction temperature range in an apparatus for thermal decomposition. [0003] Reaction tubes are exposed to high temperature atmosphere and susceptible to, for example, oxidation, carburization or nitridation by the flowing raw material gas and the like, and thus are required to have excellent resistance to them. For this reason, heat-resistant austenite alloys having excellent high temperature strength have been used for reaction tubes. [0004] A metal oxide layer is formed on the surface of a heat-resistant austenite alloy during use in a high temperature atmosphere, and this oxide layer serves as a barrier to protect base materials in the high temperature atmosphere. Meanwhile, when Cr in the base material is oxidized to form a metal oxide, which is Cr oxide (mainly composed of Cr2O3), internal oxidation may occur in a high temperature atmosphere and thus the oxide layer may be enlarged because Cr oxide is not dense and thus does not havea sufficient functionto prevent entering of oxygen or carbon. Furthermore, Cr oxide is easily peeled off in repeated cycles of heating and cooling, and even if not peeled off, since Cr oxide does not have sufficient function to prevent entering of oxygen or carbon from outside atmosphere, there is such a disadvantage that oxygen or carbon penetrates through the oxide layer to cause internal oxidization or carburization in the base material. [0005] To address this, increasing the content of Al from that of usual heat-resistant austenite alloys and forming, on the surface of a base material, an oxide layer mainly composed of alumina (Al2O3), which is dense and less likely to permeate oxygen or carbon, has been proposed (see, for example, Patent Literature 1 and Patent Literature 2). [Citation List] [Patent Literature] [0006] [Patent Literature 1] Japanese Patent Laid-Open No. 51-78612 [Patent Literature 2] Japanese Patent Laid-Open No. 57-39159 [Summary of Invention] [Technical Problem] [0007] However, an increased Al content in a reaction tube causes a reduced ductility of the material, leading to a reduction in high temperature strength. Furthermore, when, in some cases, a plurality of tubular bodies are welded to increase the total length of a reaction tube, a large Al content may reduce weldability of tubular bodies to cause weld cracking. [0008] An object of the present invention is to provide a heat-resistant alloy and a reaction tube having excellent oxidation resistance, mechanical properties such as tensile ductility, and weldability. [Solution to Problem] [0009] The heat-resistant alloy of the present invention comprises, in terms of % by mass, C: 0.35% to 0.7%, Si: more than 0% and 1.5% or less, Mn: more than 0% and 2.0% or less, Cr: 22.0% to 40.0%, Ni: 25.0% to 48.3%, Al: 1.5% to 4.5%, Ti: 0.01% to 0.6%, and the balance being Fe and inevitable impurities, wherein, when Pa = -11.1 + 28.1 × C + 29.2 × Si -0.25 × Ni - 45.6 ×Ti, and Ya = -13.75 × Al + 63.75, Pa Ya. [0054] Before the bead-on-plate test, the test surface of the specimens was smoothed by mechanical processing by a grinder. The test surface constitutes a welding groove and a part affected by heat. [0055] Furthermore, the test surface of the respective specimens was subjected to liquid penetrant testing to see that the test surface was free from cracking. [0056] The specimens which were found to have a sound test surface were subjected to a bead-on-plate test by TIG welding in the condition shown in Table 2. The bead was a straight bead, and the bead length was 50 to 100 mm. [0057] [Table 2] [0058] For the order of carrying out the present test, a test according to method A was performed, and then if defects were found in the liquid penetrant testing, a test according to method B was performed. [0059] The criteria for evaluating beads according to method A (filler metal (welding rod), not used) and method B (filler metal, used) are shown in Figure 2 and Table 3. In method B, the evaluation is "NG" even when cracks are very small. [0060] [Table 3] [0061] As the results of the above test, specimens in which no defects were found in both specimens having a thickness of 25 mm or less and specimens having a thickness of 25 mm or more according to method A were rated as "A" for cracking properties; specimens in which defects were found by method A but defects were not found by method B were rated as "B" for cracking properties; and specimens in which defects were found even by method B were rated as "C" for cracking properties. The results are shown in "Cracking properties" in Table 1. [0062] Referring to Table 1, while all of specimens Nos. 11 to 23, which were Inventive Examples, were rated as "A" or "B" for cracking properties, all of specimens Nos. 31 to 38, which were Comparative Examples, were rated as "C" for cracking properties. [0063] Comparative Examples satisfy Pa 0, and more desirably Ya> 15. [0067] The above description illustrates the present invention and should not be construed as limiting the invention according to the claims or limiting the scope of the invention. Furthermore, obviously the features of the present invention are not limited to those in Examples described above and may be modified in many ways within the technical scope described in the claims. [0068] The heat-resistant alloy of the present invention may also be applied to products which require, for example, heat resistance and oxidation resistance, such as a kiln, a retort, a burner tube and a radiant tube in addition to the reaction tube according to the above embodiments. [Reference Signs List] [0069] 10 Bead 12 Crater 14 Cracking 16 Dot defects WE Claim: [Claim 1] A heat-resistant alloy comprising, in terms of % by mass, C: 0.35% to 0.7%, Si: more than 0% and 1.5% or less, Mn: more than 0% and 2.0% or less, Cr: 22.0% to 40.0%, Ni: 25.0% to 48.3%, Al: 1.5% to 4.5%, Ti: 0.01% to 0.6%, and the balance being Fe and inevitable impurities, wherein when Pa = -11.1 + 28.1 × C + 29.2 × Si -0.25 × Ni - 45.6 ×Ti, and Ya = -13.75 × Al + 63.75, Pa