JPH022943B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a heat-resistant cast alloy used in reaction tubes and the like in the petrochemical industry, for example, a heat-resistant cast alloy used in reaction tubes for ethylene cracking furnaces. Furthermore, the present invention relates to a low carbon heat resistant cast alloy with good weldability. Conventionally, Ni and other materials have been used for the above reaction tubes, etc.
Heat-resistant cast steel containing Cr, such as ASTM HK40 material (0.4C―25Cr―20Ni―remaining Fe) and 0.1C―20Cr―
32Ni-1Nb steel is used. Among these, HK40 material is generally used in a temperature range of 700 to 1000°C. The reason for this is that when C in the material, which is dissolved in austenite during casting, is heated to high temperatures, it combines with Cr and disperses and precipitates as fine carbides, and when heated for an even longer period of time, the carbides grow and become coarser. As a result, the creep rupture strength in this state is lower than the value linearly extrapolated from the short-time data. Furthermore, 0.1C-20Cr-32Ni-1Nb steel has a creep rupture strength almost equivalent to that of HK40 material in a temperature range of 950°C or lower, despite having a low content of C, which improves creep rupture strength. This is because the above alloy contains Nb, so during casting and solidification, Nb
This is because NbC combines with C to precipitate at grain boundaries, and the NbC retards the formation and growth coalescence of grain boundary voids, which are thought to promote high-temperature creep fracture. However, such effect is 950℃
It is noticeable in the following temperature ranges, but becomes unrecognizable at temperatures above 950°C. Therefore 0.1C―
Similarly to the HK40 material, the creep rupture strength of 20Cr-32Ni-1Nb steel also decreases in the high temperature range. In addition, the heat-resistant cast steel mentioned above can be heated to a temperature of 1050℃.
It also has the disadvantage of poor carburization resistance in the temperature range exceeding . Therefore, as a heat-resistant casting alloy used for reaction tubes and the like in the petrochemical industry, there is a desire to develop a material with high creep rupture strength in a high temperature range and excellent carburization resistance. Furthermore, in the petrochemical industry, arc welding such as TIG welding, MIG welding, and shielded arc welding is used when continuously piping is used to assemble reaction tubes such as reflow march tubes. Good weldability is also required as a heat-resistant casting alloy. The present invention was obtained as a result of developing a low-carbon, heat-resistant cast alloy that satisfies such requirements.
The purpose of the present invention is to provide a heat-resistant cast alloy that has high creep rupture strength in a high-temperature region, excellent carburization resistance, and good weldability. The carburization-resistant heat-resistant casting alloy with high creep rupture strength according to the present invention has C: 0.07 to 0.25%, Si: 3.0% or less, Mn: 2.0% or less, P: 0.03% or less, and S: 0.03.
% or less, Cr: 18.0-27.0%, Ni: 20.0-40.0%,
Nb: 0.3-1.8%, W: 0.3-4.0%, Al: 0.02-0.4
%, B: 0.0005 to 0.01%, Zr: 0.02
~0.5% and/or Ti: 0.02~0.5%, and the remainder is substantially Fe. Next, the reasons for limiting the components of the cast alloy of the present invention will be explained in detail. C: 0.07 to 0.25% C combines with Nb to form eutectic carbide at grain boundaries, increasing grain boundary fracture resistance and increasing creep rupture strength. For this purpose, at least 0.07% is required to obtain high creep rupture strength, especially at temperatures above 700°C. On the other hand, if C exceeds 0.25%, its contribution to improving creep rupture strength will decrease, and embrittlement due to Cr carbide precipitation will increase, so the upper limit was set at 0.55%. Si: 3.0% or less Si, which is mixed in small amounts from molten raw materials, is an effective element because it increases the fluidity of molten steel, improves castability, and enhances the deoxidizing effect. However, if it exceeds 3.0%, it will have a negative effect on creep rupture strength.
The upper limit was set at 3.0%. Mn: 2.0% or less Mn is effective as an element that deoxidizes the molten metal, fixes the impurity element S in the molten metal, and prevents hot cracking during welding. However, if the content exceeds 2.0%, the effect is small compared to the amount added, so the upper limit was set at 2.0%. P: 0.03% or less If the P content exceeds 0.03%, the susceptibility to hot cracking during welding increases significantly, so the upper limit is 0.03%.
%. S: 0.03% or less Similar to P, S content exceeding 0.03% significantly increases the susceptibility to hot cracking during welding, so the upper limit was set at 0.03%. Cr: 18.0-27.0% Minimum operating temperature: In order to give the material oxidation resistance and high-temperature strength from a state of 700°C, it is necessary to contain at least 18.0% Cr. Furthermore, when the content exceeds 18.0%, the oxidation resistance and high temperature strength improve as the amount increases, but when the content exceeds 27.0%,
The upper limit was set at 27.0% because it becomes structurally unstable at low temperatures (below 900°C), and embrittlement due to Cr carbide precipitation becomes significant. Ni: 20.0-40.0% Ni forms an austenite phase together with Cr and Fe,
It is an element that stabilizes austenite, improves oxidation resistance, and increases high-temperature strength. In order to improve the above-mentioned oxidation resistance and high-temperature strength in a temperature range of 700°C or higher, at least 20.0% of Ni is required. When Ni is contained at 20.0% or more, the oxidation resistance and carburization resistance improve as the content increases, and the structure (particularly the degree of carbide aggregation) is stabilized in high-temperature regions.
However, since there is no significant effect on high temperature strength even if the content exceeds 40.0%, the upper limit is 40.0%.
And so. Nb: 0.3-1.8% As mentioned above, Nb combines with C during casting solidification,
Generate NbC. NbC crystallized at grain boundaries,
Increasing creep rupture resistance results in increased creep rupture strength. The effect is that the Nb content is 0.3
%, so the lower limit was set at 0.3%. As for the above-mentioned creep rupture strength, it was found from short-term investigation results that the effect becomes smaller when the Nb content exceeds 1.8%. Therefore, the upper limit is
It was set at 1.8%. Furthermore, since NbC crystallizes during solidification, supersaturated C in austenite is reduced, so precipitation of Cr carbide after high-temperature heating is reduced, and high-temperature deformation of austenite is increased. W: 0.3 to 4.0% W is dissolved in austenite and has a solid solution strengthening effect. This effect is recognized from a state where W is contained in an amount of about 0.3%, and increases as the content increases. However, if its content exceeds 4.0%, it will harden and its ductility at low temperatures will decrease, resulting in poor workability and
Weldability also deteriorates. Therefore, W was set at 0.3 to 4.0%. Al: 0.02-0.4% The effect of Al on improving creep rupture strength is small, and if Al exceeds about 0.4%, it has a negative effect on ductility at room temperature, so the upper limit of Al was set to 0.4% or less. On the other hand, when Al is contained in an amount of 0.02% or more, a surface film is formed when heated to a high temperature to prevent diffusion of C in the carburizing atmosphere, thereby improving carburization resistance. Therefore, the lower limit is 0.02
%. B: 0.0005 to 0.01% B suppresses the growth of secondary carbides generated in austenite and contributes to improving creep rupture strength. The effect is recognized from 0.0005%, but 0.01%
If it exceeds %, it will have a negative effect on weldability.
The allowable range for B was 0.0005 to 0.01%. Next, let's talk about Zr and Ti. As Zr increases, creep rupture strength improves. On the other hand, Ti is Cr formed in austenite by reheating.
It delays the growth and coarsening of carbides and improves creep rupture strength. All elements range from 0.02 to
When the content is 0.5%, the effect of improving the creep rupture strength mentioned above is observed. Therefore, the cast alloy according to the present invention contains Zr: 0.02 to 0.5% and/or Ti: 0.02 to 0.5%.
It was decided to contain 0.5%. Hereinafter, the present invention will be specifically explained with reference to Examples. Various cast steels having the compositions shown in Table 1 were melted using a high frequency induction melting furnace. In Table 1,
No. 1 to No. 3 correspond to the invention materials, which were melted as materials for various tests, and No. 4 corresponds to the invention materials, but were melt-manufactured as materials for preparing filler wire for welding. This is what I did. Furthermore, No. 11 in Table 1
and No. 12 correspond to conventional materials, which were melted as materials for each test, and No. 13 corresponds to conventional materials, but were melted as materials for preparing filler wire for welding. The above cast steel was subjected to centrifugal casting to obtain a cast steel pipe with an outer diameter of 138 mm, a wall thickness of 23.5 mm, and a length of 520 mm. Test pieces were prepared from each tube and subjected to a creep rupture strength test and a carburization resistance test. The above creep rupture test was carried out for two purposes: to compare the inventive material with conventional materials, and to compare the weld base metal and welded joint in order to examine the weldability of the inventive material. did. Note that manual TIG welding (downward welding) was used as the welding method to obtain the above-mentioned welded joint. That is, the base material made of No. 3 or No. 11 in Table 1 is subjected to a predetermined beveling process (bevel angle: 20°), the base materials are butted together, and then the base material made of No. 4 or No. 11 in Table 1 is Manual TIG using filler wire cut and stretched from No. 13 material under specified welding conditions (welding current: 90 to 150 A, welding voltage: 14 to 18 V, welding speed: 5.3 to 7.5 m/min) Welding was performed to obtain the above-mentioned welded joint. The creep rupture test was conducted based on the provisions of JIS Z 2272 (test temperature: 982°C). In addition, as a carburization resistance test, a specimen (diameter 12 mm x length 60 mm) was placed in a fixed carburizing agent (Degussa KG30) at a temperature of 1050℃ for 300 minutes.
After holding for a period of time, chips were collected at 0.25 mm intervals from the specimen surface and chemically analyzed to determine the increase in carbon content at a position 1 mm from the surface, thereby evaluating the carburization resistance. The test results are shown in Figure 1 (creep rupture characteristics of conventional material), Figure 2 (creep rupture characteristics of the invention material), Figure 3 (creep rupture characteristics of the welded joint of the invention material),
It is shown in Table 2 (carburization resistance comparison test results). In addition,
Figure 1 also shows the creep rupture characteristics of a conventional welded joint.

【table】

【table】

[Table] The test results show that although the long-term creep rupture strength of the conventional material is lower than the value linearly extrapolated from the short-term data (see Figure 1),
It can be seen that the long-term creep rupture strength of the material of the present invention does not decrease as much as the conventional material (see Fig. 2). In addition, the welded joints of conventional materials are approximately
Although it shows a 10% decrease in strength (see Figure 1),
The welded joint of the material of the present invention had a strength equivalent to that of the base metal, indicating that the material of the present invention had good weldability. Furthermore, in terms of carburization resistance, the carbon content of the present invention material at a position 1 mm from the surface is less than 50% compared to conventional materials (see Table 2), and the carburization resistance has been significantly improved. I found out that there was. As detailed above, the heat-resistant cast alloy of the present invention has higher creep rupture strength in high-temperature regions than conventional heat-resistant cast steels containing Ni and Cr, has excellent carburization resistance, and is a material with good weldability. So,
It is ideal as a material for reflow marching tubes and cracking tubes used in the petrochemical industry. It is also extremely useful as a material for various steel-related equipment members, such as hearth rollers and heat treatment trays.

[Brief explanation of drawings]

Fig. 1 is a graph showing the creep rupture properties of the conventional material, Fig. 2 is a graph showing the creep rupture properties of the inventive material, and Fig. 3 is a graph showing the creep rupture properties of the welded joint of the inventive material compared to the base metal. This is a graph shown.

Claims (1)

[Claims] 1 C: 0.07-0.25%, Si: 3.0% or less, Mn: 2.0
% or less, P: 0.03% or less, S: 0.03% or less, Cr:
18.0~27.0%, Ni: 20.0~40.0%, Nb: 0.3~1.8
%, W: 0.3-4.0%, Al: 0.02-0.4%, B:
A carburizing-resistant, heat-resistant casting alloy with high creep rupture strength, containing Zr: 0.02-0.5% and/or Ti: 0.02-0.5%, the balance being substantially Fe.