JPS583015B2 - Heat treatment method for seamless nickel-chromium iron alloy pipes formed from centrifugally cast pipes - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention prevents cracks that often occur during solution heat treatment after the drawing process, in the process of making a seamless pipe by drawing a so-called alloy 800 cast pipe as a blank pipe. Concerning the provision of a new method of shaving.

Conventionally, seamless pipes made of so-called Alloy 800, which have been used for reaction tubes, etc., are first formed into a raw pipe by drawing, extrusion, machining, etc. after forging an ingot, and then formed into the final purpose by drawing or extrusion processing. Manufacturing methods have been adopted to finish the product.

Alloy 800 means JISMCF800 or NC.
It means F800H material, and its specific chemical composition range is shown in the table below (see JIS G4903 (1981)).
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By the way, the present inventors have already proposed a manufacturing method in which a centrifugally cast pipe of the same material is directly drawn as a blank pipe in order to shorten the manufacturing process of this type of seamless nickel-chromium iron alloy pipe.

In other words, the above method uses centrifugal force casting to produce a raw tube at once from a conventional ingot through multiple processes, and then draws the same raw tube, thereby creating a cast structure that is generally coarser than a forged structure. This method aims to improve the strength at room temperature, increase and stabilize the creep rupture strength at high temperatures, and form the pipe into the desired dimensions by creating a microstructure equivalent to that of forged or pressed products.The attached diagram shows an overview of the drawing process. This is what is shown.

That is, the distal end 1' of the alloy 800 centrifugally cast tube 1 is grasped by the gripping member 3, and the cast tube 1 is pulled out through the die 2.

The tube body pulled out in the above manner is usually heated to 1100℃
Cracks often occur during solution treatment, which involves heating for 30 minutes to 1 hour and then cooling with water, and there has been a strong desire to establish a method for preventing these cracks.

The present invention has been made in response to the above-mentioned desires,
Its characteristics are JIS NCF800 or NCF8
When a centrifugally cast pipe of 00H material is used as a raw pipe and subjected to cold drawing and then solution treatment, if the average grain size of the crystal grains of the raw pipe exceeds 0.8 mm, after the drawing process. The solution treatment is carried out after a preliminary heat treatment in which the tube is heated to 500 to 1100°C and slowly cooled.

To explain the present invention below, it is first necessary to understand under what circumstances the cracks occur during the solution treatment, and the size of the crystal grains of the cast pipe,
The presence or absence of cracks was investigated for samples with various drawing rates.

The results are shown in Table 1 below. The size of the crystal grains in the same table is (major axis + minor axis) / 2 (mm), where the major axis is the crystal grain size measured in the wall thickness direction of the tube, and the minor axis is the crystal grain size measured in the circumferential direction of the tube. According to ASTM-E112.

Note that the solution treatment after drawing was performed at 1100° C. for 1 hr, followed by water cooling.

The chemical components of the cast pipes used in the above measurement tests are shown in the table below.

As is clear from the above table, the occurrence of cracks cannot be recognized from the difference in processing rate, but it has been found that it depends on the size of the crystal grains in the cast pipe. The size was determined to be 0.8 mm or more in average diameter of the major axis and minor axis.

The reason why a cast pipe with an average crystal grain size of 0.8 mm is targeted in the present invention is based on the above-mentioned experimental results. As a result of various experiments to prevent this, it was found that it is effective to heat the material to 500° C. or higher and slowly cool it after the above-mentioned drawing process.

Table 2 below shows the results of investigating the soundness of drawn cast tubes heated to various temperatures and slowly cooled (preliminary heat treatment) after solution treatment. As mentioned above, the size is the average particle diameter of the major axis and minor axis, which is 1.
．． It was 65mm.

The drawing processing rate for cast raw pipe is 15%,
After heating, the mixture was slowly cooled down to room temperature by cooling in the furnace.

The solution treatment was performed at 1100° C. for 1 hr, followed by water cooling.

As can be seen from the table above, heating below 500°C does not have a crack prevention effect, so the lower limit of heating for preheating was set at 500°C, but the upper limit of 1100°C
For this reason, the temperature is the solution treatment temperature, and if it exceeds this temperature, the coarsening rate of crystal grains becomes excessive, resulting in a decrease in quality. Heating takes less than 1 hour.

The present invention is as described above, and the cracks that occur during the solution treatment in the production of seamless pipes by drawing the centrifugally cast pipes of the so-called Alloy 800 material identified above can be fixed at a specific temperature as described above. By inserting a simple process of heating and slow cooling before solution treatment, it is possible to manufacture sound pipes as shown in Table 2 above. It can be said that the invention has dramatically improved the value of the manufacturing method of Alloy 800 seamless pipes using centrifugally cast pipes as the raw pipe, and the process is much simpler than the conventional manufacturing process of Alloy 800 seamless pipes from ingots. The advantages of this method are extremely large, such as shortening of time and cost advantages, and the method of the present invention can also be considered to be used when manufacturing seamless pipes by drawing or other processing from cast pipes made of other stainless steels, not just Alloy 800. Therefore, the industrial value of the present invention is significant.

[Brief explanation of the drawing]

The attached figure is a diagram showing an outline of drawing processing of a centrifugally cast tube using Alloy 800. DESCRIPTION OF SYMBOLS 1... Centrifugal force casting tube of alloy 800, 1'... Tip part (gripping part) of casting tube, 2... Dice, 3... Gripping member.

Claims (1)

[Claims] 1. When using a centrifugally cast tube made of JIS NCF800 or NCF800H material as a blank tube and subjecting it to cold drawing and then solution treatment, if the average grain size of the crystal grains of the blank tube exceeds 0.8 mm, drawing is not allowed. 50 tubes after processing
A heat treatment method for a seamless nickel-chromium iron alloy pipe formed from a centrifugally cast pipe as a raw pipe, characterized in that the solution treatment is performed after preliminary heat treatment of heating to 0 to 1100°C and slow cooling.