JPH0382711A - Method for cooling martensitic stainless steel tube - Google Patents

Abstract

PURPOSE:To improve the quality of a steel tube without causing defects, such as quenching crack, by carrying out cooling after heating at a cooling velocity in a specific range at the time of subjecting a martensitic stainless steel tube to hardening treatment. CONSTITUTION:Hardening is applied to a steel tube in which a martensitic stainless steel of 9Cr, 13Cr, etc., is used as a stock by spraying cooling water to the steel tube through a nozzle at about 800 deg.C or below to carry out accelerately cooling at 1-20 deg.C/sec cooling rate, by which the quality of the steel tube can be improved by means of hardening treatment without causing defects, such as quenching crack, to the steel tube. By this method, the quality of the martensitic stainless steel tube can be improved without requiring long time for hardening treatment.

Description

[Detailed Description of the Invention] "Objective of the Invention" (Industrial Application Field) The present invention relates to a method for cooling martensitic stainless steel pipes, which efficiently cools martensitic stainless steel pipes and produces high-quality products. It is about the method to obtain it.

(Prior Art) When using martensitic stainless steel pipes such as 9Cr and 13Cr for oil country tubular goods etc., it is necessary to employ a heat treatment quenching method. However, if water quenching (cooling rate of 40°C or more) is used, quench cracking and bending will occur, so air cooling or a similar cooling method with a cooling rate of less than 1°C/see is recommended. It is assumed that the

(Problem to be Solved by the Invention) However, when using the conventional method as described above, the cooling time required to cool the pipe section, which is at around 800°C or more, reaches 1 to 3 hours, which is inconvenient. Be efficient. Further, a large cooling space is required, and under conditions where the cooling space is limited, the cooling processing efficiency is inevitably very low, and if the tube has a large wall thickness, appropriate quenching cannot be achieved.

In order to avoid such disadvantages, water quenching as described above may be considered, but even if the conventional water quenching method is used for cooling, 100% quench cracking will occur, making it impossible to obtain the desired product.

``Structure of the Invention'' (Means for Solving the Problems) The present invention was created after studying to improve the disadvantages and shortcomings of the conventional products as described above, and the present invention was created by examining and improving the disadvantages and shortcomings of the conventional products as described above. This is a method for cooling martensitic stainless steel pipes, which is characterized by accelerated cooling at a cooling rate of 20° C./sec.

(Function) The cooling rate is 1 to b, thereby significantly shortening the cooling time. In other words, the tube section and the elapsed time from the start of cooling are as shown in Figure 1 using the conventional method of air cooling at less than 1°C/sec, which is at least 1 hour or more, and in some cases up to 3 hours. By employing the above-mentioned accelerated cooling, the cooling time becomes as shown by the curve a in FIG. 1, and is generally significantly shortened to about one to several minutes.

Therefore, in the case of the standard cooling time of 2 hours in the conventional method, if the cooling space is narrow, the efficiency will be low as shown by the solid line in Figure 2, but in the case of accelerated cooling as described above, the efficiency will be significantly lower as shown by the broken line in the figure. Increases efficiency.

Furthermore, as shown in Fig. 3, in the case of the conventional air cooling method, if the wall thickness is large and the cooling rate is reduced, proper quenching may not be obtained. Proper quenching can be obtained.

In other words, if the cooling rate is less than 1°C/sec, the efficiency improvement described above cannot be achieved accurately;
When the temperature exceeds ec, the possibility of quench cracking increases rapidly, and by setting the upper limit to 20° C./sec, such a tendency can be effectively avoided.

Note that the cooling rate as described above is from 800℃ to 50℃ for the pipe section.
shall be applied within the scope of.

(Example) To further explain the above-mentioned present invention, the present inventors have achieved high efficiency and sufficient hardenability for martensitic stainless steel pipes such as 9Cr and 13Cr, while also preventing quench cracking. As a result of repeated studies on cooling using a water-cooling method that does not require cooling, we decided to spray cooling water from a nozzle from the top of the pipe to reduce the cooling rate and achieve uniform cooling compared to the conventional water-quenching method. We considered cooling the pipe by adding cooling water evenly over the entire surface of the pipe in order to prevent this from occurring, and confirmed that the above objective could be appropriately achieved by cooling the pipe from around 800°C or below. .

The cooling start temperature is preferably as low as possible in order to prevent quench cracking, but as high as possible in order to increase efficiency, it is generally 800°C or less. The lower limit is appropriately selected in the range of about 50°C to 100°C depending on the operating conditions in each case.

That is, the amount of water can be easily controlled by spraying cooling water with a nozzle, and the cooling rate can be controlled by controlling the amount of water. Figure 4 shows the cooling rate ('C/5ec) when cooling a stainless steel pipe with a wall thickness of 10 to 30 cm for 13Cr material.
The relationship between and cooling water flow density (l/win-nf) is shown below.
It is clear that the cooling rate can be controlled approximately accurately by controlling the amount of cooling water.

When spraying cooling water with the nozzle, it is preferable to vibrate the nozzle in the axial direction, and this relationship is shown separately in Fig. 5. In the case of no amplitude as shown in Fig. In contrast, when the amplitude shown in FIG. 3B is taken, there is almost no variation. In order to uniformly add cooling water to the entire surface of the tube, it is preferable to use a nozzle while rotating the tube on a turning roller.

Specifically, Figure 6 shows the results of examining the relationship between the incidence of sinter cracking and the cooling rate with n=20 in each plot by varying the cooling rate for a 13Cr stainless steel pipe. While it can be said that the incidence of quench cracking is zero up to 20°C/sec, quench cracking occurs when the rate exceeds 20°C/sec, and the rate of occurrence increases rapidly above 25°C. sec, the quench crack occurrence rate is around 50%. Also, the cooling rate difference for uniform cooling is ±30
Approximately %.

"Effects of the Invention" According to the present invention as explained above, the cooling time required for cooling during quenching can be significantly shortened and processing can be carried out efficiently, and a stable quenching effect can thereby be obtained.
Furthermore, it is possible to provide a high-quality martensitic stainless steel pipe by appropriately preventing quench cracking, and this invention is industrially highly effective.

[Brief explanation of drawings]

The drawings show the technical contents of the present invention, and Fig. 1 is a chart showing the relationship between the pipe method and the elapsed time from the start of cooling for the conventional method and the method of the present invention, and Fig. 2 is the same. Figure 3 is a diagram comparing the cooling efficiency of the conventional method and the method of the present invention. Figure 3 is a diagram comparing the quenching effect of the conventional method and the method of the present invention. Figure 4 is a diagram comparing the cooling efficiency of the conventional method and the method of the present invention. Figure 5 is a diagram showing the relationship between water flow density and cooling rate. Figure 5 is a diagram showing variations in pipe method depending on the presence or absence of axial amplitude of the cooling water nozzle. Figure 6 is a diagram showing the relationship between cooling rate and quench cracking incidence. This is an organized chart. The first circle

Claims (1)

[Claims] martensitic stainless steel pipe at 1 to 20℃/sec
A cooling method for martensitic stainless steel pipes characterized by accelerated cooling at a cooling rate of .