Apparatus and method for precise induction hardening of inner wall of large-diameter straight and curved steel pipes

Abstract

The present invention discloses a device and method for precise induction hardening of the inner wall of a large-diameter straight or curved steel pipe, comprising: an induction heater, arranged inside the steel pipe, for induction heating the steel pipe; a built-in nozzle, arranged inside the steel pipe, for spraying cooling water on the inner wall of the steel pipe; an external nozzle, arranged outside the steel pipe, for spraying cooling water on the outer wall of the steel pipe; a chain rope, for pulling the induction heater, and connecting the induction heater and the built-in nozzle. The present invention solves the problem that it is difficult to balance the wear resistance and toughness of steel pipes in traditional heat treatment, and effectively prolongs the actual service life of large-diameter medium transport steel pipes.

Description

Technical Field

[0001] The invention relates to the technical field of steel pipe manufacturing, and more specifically to a device and method for precise induction quenching of the inner wall of large-diameter straight and curved steel pipes. Background Art

[0002] In the steel pipe industry, large-diameter straight seam welded pipes formed by UOE are generally used in the field of fluid medium transportation such as oil, gas, slurry, and even ore, requiring the inner wall of the steel pipe to have good wear resistance so that the steel pipe can have a longer service life. In the process of using pipeline steel, it is not difficult to imagine that due to the long-term scouring and abrasion of the inner wall of the steel pipe by the flowing medium, especially the micro-abrasion caused by the flowing medium containing more solid particles, the wear process of the steel pipe will undoubtedly be greatly accelerated, and the wall thickness of the steel pipe at the bending position will be thinned faster. Finally, the bending part of the steel pipe is often worn out first, causing the entire steel pipe to fail and be scrapped. If the medium leakage is not discovered in time, it may also cause major safety accidents and environmental pollution, resulting in great economic losses.

[0003] Existing wear-resistant technologies for steel pipes can be roughly divided into three categories: (1) adjusting the alloy content of the steel pipe body to increase the wear resistance of the material matrix, such as increasing the Si and Mn content in molten steel in Chinese Patent 201510163300.6 and adding Zr, Y and other alloy elements to steel in Chinese Patent 201410018588.3, ​​which increases the material cost of the steel pipe; (2) preparing a wear-resistant layer on the inner wall of the steel pipe or laying a wear-resistant lining, such as Chinese Patent 20158005668 5.4 Gas nitriding, China Patent 201210242700.2 double-layer composite pipe, etc., the preparation process is relatively complicated, and the manufacturing cost increases exponentially; (3) Hardening heat treatment improves the hardness of steel pipes, and takes advantage of the high efficiency and low cost of induction heat treatment, such as China Patent 200910012360.2 steel pipe induction quenching and tempering, China Patent 201610063585.0 steel pipe scanning induction quenching, which is difficult to apply to the heat treatment of bent steel pipes. For bent steel pipes, China Patent 201611114823.2 uses a push rod mechanism to allow the bent steel pipe to intermittently enter the external heater for heating and insulation, and then exit the furnace for air cooling, which is used in the normalizing heat treatment of large P91 type bent pipes in power stations. Although there are various wear-resistant heat treatment technologies, in general, the wall thickness of steel pipes is relatively thin, and the thermal conductivity of carbon steel is relatively high. The temperature difference between the inner and outer walls of steel pipes in existing quenching heat treatments, including induction heat treatment processes, is generally relatively small. The temperature difference between the inner and outer walls of medium-frequency induction heating steel pipes is only 50°C to 100°C. During water spray cooling, the steel pipe is hardened throughout the wall thickness direction. Although the wear resistance of the steel pipe can be improved, the plasticity and toughness cannot be guaranteed, and it is difficult to achieve the ideal hardness gradient distribution characteristics of "high inside and low outside". This greatly limits the actual use and promotion of UOE wear-resistant welded pipes that have been locally hardened and heat treated.

[0004] Therefore, it is still extremely important to further seek continuous and efficient quenching heat treatment technology for large-diameter UOE wear-resistant welded pipes that can achieve precise hardening of the inner wall of the steel pipe, is preferably suitable for a variety of steel pipe shapes, and is green, energy-saving and environmentally friendly. Summary of the invention

[0005] In view of the above-mentioned defects existing in the prior art, the purpose of the present invention is to provide a precise induction quenching device and method suitable for the inner wall of large-diameter straight and curved steel pipes, so as to solve the problem that it is difficult to take into account both the wear resistance and the strength and toughness of steel pipes in traditional heat treatment, and effectively extend the actual service life of large-diameter medium transporting steel pipes.

[0006] To achieve the above object, the present invention adopts the following technical solution:

[0007] On the one hand, a device for precise induction hardening of the inner wall of large-diameter straight and curved steel pipes, comprising:

[0008] An induction heater is arranged inside the steel pipe and is used to induction heat the steel pipe;

[0009] A built-in nozzle is arranged inside the steel pipe and is used to spray cooling water on the inner wall of the steel pipe;

[0010] An external nozzle is arranged outside the steel pipe and is used to spray cooling water onto the outer wall of the steel pipe;

[0011] A chain rope is used for pulling the induction heater and connecting the induction heater and the built-in nozzle.

[0012] Preferably, the induction heater and the built-in nozzle are both provided with support rods;

[0013] The support rod is provided with a spherical roller which contacts the inner wall of the steel pipe.

[0014] Preferably, the spherical roller is arranged on the support rod through a spring.

[0015] Preferably, the chain rope, the support rod and the spherical roller are made of non-magnetic and heat-resistant stainless steel.

[0016] Preferably, the gap D1 between the outer surface of the coil winding on the induction heater and the inner wall of the steel pipe is 20-40 mm.

[0017] Preferably, the gap D2 between the induction heater and the built-in nozzle is 100-350 mm.

[0018] On the other hand, a method for precise induction hardening of the inner wall of large-diameter straight and curved steel pipes:

[0019] The steel pipe is placed still, and the steel pipe is induction heated by the induction heater in the inner wall precise induction quenching device;

[0020] The induction heater is pulled by the chain rope to move inside the steel pipe, and the induction heater is pulled by the chain rope to move together with the built-in nozzle;

[0021] The internal nozzle sprays cooling water to the inner wall surface of the steel pipe, and the external nozzle sprays cooling water to the outer wall surface of the steel pipe, so that the steel pipe is kept within a set temperature and meets the quenching hardness.

[0022] Preferably, the power P of the induction heater is between 600 and 1000 kW, and the frequency f is between 1000 and 10000 Hz.

[0023] Preferably, the quenching temperature Tq within the range of 1 / 4 to 1 / 2 thickness of the inner wall of the steel pipe is between 950 and 1050°C, and the quenching temperature Te of the outer wall of the steel pipe is between 200 and 600°C;

[0024] The quenching hardness within the range of 1 / 4 to 1 / 2 thickness of the inner wall of the steel pipe is between 30 and 50 HRC, and the quenching hardness within the range of 1 / 2 to 3 / 4 thickness of the outer wall of the steel pipe is between 20 and 25 HRC.

[0025] Preferably, the water flow rate L1 of the built-in nozzle is between 50 and 80 m 3 / h, the water flow rate L2 of the external nozzle is between 30 and 60m 3 / h;

[0026] The cooling water pressure of the internal nozzle and the external nozzle is between 0.4 and 0.8 MPa.

[0027] The present invention provides a precise induction quenching device and method for the inner wall of large-diameter straight and curved steel pipes, which can accurately achieve the ideal hardness gradient distribution from inside to outside, with the inner wall of the steel pipe being hardened and the outer wall not being hardened, and the wear resistance and plasticity and toughness of the conveying steel pipe body are simultaneously improved. Simple adjustment of the heat treatment equipment can be applicable to the continuous hardening heat treatment of the inner wall of straight steel pipes and curved steel pipes, and it is green, energy-saving, environmentally friendly, and efficient, fundamentally extending the service life of large-diameter UOE steel pipes. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the structure of the inner wall precise induction quenching device of the present invention;

[0029] Figure 2 It is a schematic diagram of the temperature distribution characteristics of the inner and outer walls of the steel pipe;

[0030] Figure 3Schematic diagram of the use of the inner wall precision induction quenching device of the present invention in Example 3. DETAILED DESCRIPTION

[0031] In order to better understand the above technical solution of the present invention, the technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0032] Combination Figure 1 to Figure 2 As shown, the present invention provides a precise induction quenching device for the inner wall of a large-diameter straight or curved steel pipe, comprising:

[0033] The induction heater 4 is arranged inside the steel pipe 1 and is used for induction heating the steel pipe 1;

[0034] The built-in nozzle 5 is arranged inside the steel pipe 1 to spray cooling water on the inner wall of the steel pipe 1;

[0035] The external nozzle 3 is arranged outside the steel pipe 1 and is used to spray cooling water on the outer wall of the steel pipe 1;

[0036] The chain rope 2 is used to pull the induction heater 4 and connect the induction heater 4 and the built-in nozzle 5.

[0037] The induction heater 4 and the built-in nozzle 5 are positioned by evenly distributing 3 to 5 support rods 7 along the circumference. The head of the support rod 7 is provided with a spherical roller 6. The spherical roller 6 enables the support rod 7 to adaptively retract and extend according to the shape change of the inner wall of the steel pipe 1 (such as welds, bends, etc.) through a spring 8, thereby ensuring that the induction heater 4 and the built-in nozzle 5 can pass smoothly inside the curved steel pipe 1 and always remain at the symmetrical center position of the steel pipe 1.

[0038] During heat treatment, the steel pipe 1 remains stationary, and the built-in induction heater 4 is pulled by the flexible chain rope 2 to move inside the steel pipe 1. The induction heater 4 and the built-in nozzle 5 are also connected by the chain rope 2. The induction heater 4 and the built-in nozzle 5 located behind the induction heater 4 are pulled by the chain rope 2 to move synchronously inside the steel pipe 1, and the movement speed V=0.1~0.5m / min.

[0039] The chain rope 2, the support rod 7 and the spherical roller 6 are made of non-magnetic heat-resistant stainless steel.

[0040] The external nozzle 3 is arranged around the outer circumference of the steel pipe 1. During the heat treatment process, the external nozzle 3 can remain stationary. When heat treating a long large-diameter straight pipe, the external nozzle 3 can also move in coordination with the movement of the induction heater 4 to reduce the complexity of the arrangement and control of the external nozzle 3.

[0041] The present invention also provides a method for precise induction quenching of the inner wall of large-diameter straight and curved steel pipes:

[0042] The steel pipe 1 is placed still, and the steel pipe 1 is induction heated by the induction heater 4 in the inner wall precise induction quenching device of the present invention;

[0043] The induction heater 4 is pulled by the chain rope 2 to move inside the steel pipe 1, and the induction heater 4 is pulled by the chain rope 2 to move together with the built-in nozzle 5;

[0044] The internal nozzle 5 sprays cooling water on the inner wall surface of the steel pipe 1, and the external nozzle 3 sprays cooling water on the outer wall surface of the steel pipe 1, so that the steel pipe 1 is kept within the set temperature and meets the quenching hardness.

[0045] The power supply power of the induction heater 4 is P = 600 ~ 1000kW, and the frequency f = 1000 ~ 10000Hz. It can be known from the induction heating theory that under the action of the "proximity" effect of the alternating electromagnetic field, a large amount of eddy current is induced in the wall of the steel pipe 1, and the eddy current flows in the wall of the steel pipe 1 There is an "ohmic heat" effect, and the inner wall of the steel pipe 1 is inductively heated by this resistance heating effect, so that the temperature within the range of 1 / 4 to 1 / 2 thickness of the inner wall of the steel pipe 1 is rapidly increased to the quenching temperature Tq = 950 ~ 1050 ° C in a relatively short time, and the internal microstructure of the steel pipe 1 undergoes austenitization and a small amount of carbide is solid-dissolved, and then a martensitic (bainitic) phase transformation occurs during the subsequent internal rapid cooling and quenching process, forming a wear-resistant hardened inner layer with high hardness. At the same time, the outer wall of the steel pipe 1 is always kept at a relatively low temperature below Te = 200-600 ° C under the cooling effect of the aerosol sprayed by the external nozzle 3, and no austenite and martensite transformation occurs, which ensures that the outer wall of the steel pipe 1 has high plasticity and toughness. The temperature field distribution characteristics of the inner wall thickness of the steel pipe 1 during the induction heating process are as follows: Figure 2 As shown. Through the above-mentioned inner wall induction quenching, the inner wall hardness of 1 / 4 to 1 / 2 of the steel pipe 1 is improved, and the surface quenching hardness is 30 to 50HRC, so that the inner wall of the steel pipe 1 maintains sufficient wear resistance, avoids the steel pipe 1 being worn out by the conveying medium too early, and prolongs the service life of the steel pipe 1; the outer wall hardness of 1 / 2 to 3 / 4 of the steel pipe 1 maintains the original 20 to 25HRC, so that the steel pipe 1 maintains sufficient plasticity and toughness, avoids the steel pipe 1 in the subsequent processing and use process may crack or even break and even cause the conveying medium leakage problem, also prolongs the service life of the steel pipe 1; and, because the temperature of a certain thickness range of the outer wall of the steel pipe 1 is always low, it is less affected by the induction heating of the inner wall, and the steel pipe 1 maintains a high strength during the heat treatment process, which largely eliminates people's concerns about the high temperature softening problem that often exists in the induction heating process of thin-walled steel pipes, and effectively reduces the heat treatment deformation of the steel pipe workpiece.

[0046] The induction heater 4 generally includes parameters such as the coil structure, the shape of the coil winding and the number of turns, so that the outline of the induction coil is cylindrical and matches the shape of the inner wall of the steel pipe 1, and the gap D1 between the outer surface of the coil winding and the inner wall surface of the steel pipe 1 is 20-40 mm, so that the induction heater 4 can smoothly move at a uniform speed inside the steel pipe 1 without being blocked by protrusions such as welds, and can generate an alternating electromagnetic field with uniform distribution and high magnetic field intensity on the inner wall of the steel pipe 1, thereby improving the energy conversion efficiency of induction heating.

[0047] The frequency f of the alternating current passed through the induction coil (i.e., the induction heating frequency) makes the skin depth of the electromagnetic field on the inner wall of the steel pipe 1 account for 1 / 4 to 3 / 4 of the thickness of the steel pipe wall. With the cooperation of external cooling, the quenching of 1 / 4 to 1 / 2 thickness of the inner wall of the steel pipe is accurately achieved, while the outer wall is not excessively affected by the quenching of the inner wall. Of course, the efficiency of the power supply equipment must also be considered. The recommended power supply frequency f=1000~10000Hz, and it is best to select the optimal actual use frequency value on the lower limit.

[0048] The cooling intensity of the cooling water sprayed by the internal nozzle 5 and the external nozzle 3 on the inner and outer walls of the steel pipe 1 (the cooling water flow rate L1 of the internal nozzle 5 and the cooling water flow rate L2 of the external nozzle 3), including the distance D2 between the induction heater 4 and the internal nozzle 5, is to make the 1 / 4 to 1 / 2 thickness range of the inner wall of the steel pipe 1 be heated to the quenching temperature Tq under the induction heating of the induction heater 4, and at the same time, the 1 / 2 to 3 / 4 thickness range of the outer wall of the steel pipe 1 is always kept below the low Te temperature during the induction heating process. The recommended D2 is 100 to 350 mm, and L1 is 50 to 80 m 3 / h, L2=30~60m 3 / h, cooling water pressure 0.4~0.8MPa.

[0049] Example 1

[0050] This embodiment is a thin-wall quenching of a bent pipe. For a large-diameter bent steel pipe 1 with a diameter of 1000 mm and a wall thickness of 20 mm, refer to Figure 1 During the heat treatment, the steel pipe 1 is stationary and placed vertically. The induction heater 4 is pulled by the chain rope 2 to move inside the steel pipe 1. The induction heater 4 is pulled by the chain rope 2 to move the built-in nozzle 5 behind it. The moving speed of the induction heater 4 and the built-in nozzle 5 inside the curved steel pipe 1 is V = 0.5m / min. The power supply power of the induction heater 4 is P = 850kW, the current frequency f = 6000Hz, the skin effect depth δ = 8mm, and the cooling water flow rate of the built-in nozzle 5 is L1 = 50m 3 / h, the cooling water is sprayed on the inner wall of the induction-heated high-temperature steel pipe 1 and then flows out from the lower port of the elbow by gravity. The induction heating control temperature of the inner wall surface of the steel pipe 1 is Tq = 1000 ° C. Under the cooling effect of the aerosol sprayed by the external nozzle 3, the cooling water flow rate of the external nozzle 3 is L2 = 40m3 / h, the outer surface of the steel pipe 1 is always kept below 300-500°C, the quenching hardness of the inner wall of the steel pipe within about 1 / 3 of the wall thickness range (6-8mm) is 30-40HRC, and the hardness of the outer wall of the steel pipe within about 2 / 3 of the wall thickness range (12-14mm) is maintained at 20-25HRC.

[0051] Example 2

[0052] This embodiment is a thick wall hardening of a bent pipe. For a large-diameter bent steel pipe 1 with a diameter of 1000 mm and a wall thickness of 20 mm, refer to Figure 1 During the heat treatment, the steel pipe 1 is stationary and placed vertically. The induction heater 4 is pulled by the chain rope 2 to move inside the steel pipe 1. The induction heater 4 is pulled by the chain rope 2 to move the built-in nozzle 5 behind it. The moving speed of the induction heater 4 and the built-in nozzle 5 inside the curved steel pipe 1 is V = 0.1m / min. The power supply power of the induction heater 4 is P = 650kW, the current frequency f = 2000Hz, the skin effect depth δ = 13mm, and the cooling water flow rate of the built-in nozzle 5 is L1 = 75m 3 / h, the cooling water is sprayed on the inner wall of the high-temperature steel pipe 1 and then flows out from the lower port of the elbow by gravity. The inner wall surface of the steel pipe 1 is induction heated to control the temperature Tq = 1000 ° C. Under the cooling effect of the aerosol sprayed by the external nozzle 3, the cooling water flow rate of the external nozzle 3 is L2 = 45m 3 / h, the outer surface of the steel pipe 1 is always kept below 300-500°C, the quenching hardness of the inner wall of the steel pipe within about 1 / 2 wall thickness range (10mm) is 30-40HRC, and the hardness of the outer wall of the steel pipe within about 1 / 2 wall thickness range (10mm) is maintained at 20-25HRC.

[0053] Example 3

[0054] This embodiment is the induction hardening of a straight pipe. For a large-diameter straight steel pipe 1 with a diameter of 1000 mm, a wall thickness of 20 mm, and a length of 10 m, refer to Figure 3 During heat treatment, the steel pipe 1 is stationary and tilted, with the angle θ between the center of the steel pipe 1 and the horizontal line being 15°. The induction heater 4 is pulled by the chain rope 2 to move inside the steel pipe 1. The induction heater 4 is pulled by the chain rope 2 to move the built-in nozzle 5 behind it. The moving speed of the induction heater 4 and the built-in nozzle 5 inside the straight pipe 1 is V=0.5m / min. The power supply of the induction heater 4 is P=1000kW, the current frequency is f=2000Hz, the skin effect depth is δ=13mm, and the cooling water flow rate of the built-in nozzle 5 is L1=80m 3 / h, after the cooling water is sprayed, it flows out from the lower port of the inclined straight pipe with the help of gravity. The inner wall surface of the steel pipe 1 is induction heated to control the temperature Tq=1000℃. Under the cooling effect of the aerosol sprayed by the external nozzle 3, the cooling water flow rate L2 of the external nozzle 3 is 60m3 / h, and the outer surface of the steel pipe 1 is always maintained below 300-500℃. The quenching hardness of the inner wall of the steel pipe 1 is 30-40HRC within the range of about 1 / 3 of the wall thickness (7mm), and the hardness of the outer wall of the steel pipe is maintained at 20-25HRC within the range of about 2 / 3 of the wall thickness (13mm).

[0055] To sum up, the present invention is suitable for the induction quenching heat treatment of the inner wall of various straight / bent steel pipes through the design of built-in induction heater, built-in cooling nozzle, external cooling nozzle, flexible drag chain, retractable support rod and other structures, and can more accurately realize the hardness gradient distribution characteristics of high inside and low outside, improve the wear resistance of the inner wall and the strength and toughness of the outer wall, the equipment is simple, the functions are diverse, the adjustment is convenient, and it is green, energy-saving, environmentally friendly, and efficient. It can be promoted and used in the field of induction heat treatment technology of large-diameter steel pipes such as straight seam welded pipes, spiral welded pipes, large bent pipes for medium transportation, and has broad prospects.

[0056] Those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention, and are not intended to limit the present invention. As long as they are within the spirit of the present invention, any changes or modifications to the above embodiments will fall within the scope of the claims of the present invention.

Claims

1. A precise induction hardening device for the inner wall of large-diameter straight and curved steel pipes, characterized in that: include: An induction heater is arranged inside the steel pipe and is used for induction heating the steel pipe; A built-in nozzle is arranged inside the steel pipe and is used to spray cooling water on the inner wall of the steel pipe; An external nozzle is arranged outside the steel pipe and is used to spray cooling water onto the outer wall of the steel pipe; A chain rope is used to pull the induction heater and connect the induction heater and the built-in nozzle, The induction heater and the built-in nozzle are both provided with support rods; The support rod is provided with a spherical roller in contact with the inner wall of the steel pipe. The spherical roller is arranged on the support rod through a spring. The external nozzle is arranged around the outer circumference of the steel pipe. The inner wall precision induction quenching device applicable to large-diameter straight and curved steel pipes performs the following inner wall precision induction quenching method: The steel pipe is placed still, and the steel pipe is induction heated by the induction heater in the inner wall precise induction quenching device; The induction heater is pulled by the chain rope to move in the steel pipe, and the induction heater pulls the built-in nozzle to move in the steel pipe together with the chain rope; The internal nozzle sprays cooling water on the inner wall surface of the steel pipe, and the external nozzle sprays cooling water on the outer wall surface of the steel pipe, so that the steel pipe is kept within the set temperature and meets the quenching hardness. The power P of the induction heater is between 600 and 1000 kW, and the frequency f is between 1000 and 10000 Hz. The quenching temperature Tq within the range of 1 / 4 to 1 / 2 thickness of the inner wall of the steel pipe is between 950 and 1050°C, and the quenching temperature Te of the outer wall of the steel pipe is between 200 and 600°C; The quenching hardness within the range of 1 / 4 to 1 / 2 thickness of the inner wall of the steel pipe is between 30 and 50 HRC. The water flow rate L1 of the built-in nozzle is 50~80m 3 / h, the water flow rate L2 of the external nozzle is between 30 and 60m 3 / h; The cooling water pressure of the internal nozzle and the external nozzle is between 0.4 and 0.8 MPa.

2. The device for precise induction hardening of inner wall of large-diameter straight and curved steel pipes according to claim 1 is characterized in that: The chain rope, the support rod and the spherical roller are made of non-magnetic and heat-resistant stainless steel.

3. The device for precise induction hardening of inner wall of large-diameter straight and curved steel pipes according to claim 1 is characterized in that: The gap D1 between the outer surface of the coil winding on the induction heater and the inner wall of the steel pipe is 20-40 mm.

4. The device for precise induction hardening of inner wall of large-diameter straight and curved steel pipes according to claim 1 is characterized in that: The gap D2 between the induction heater and the built-in nozzle is 100-350 mm.

5. A method for precise induction hardening of the inner wall of large-diameter straight and curved steel pipes, characterized in that: The steel pipe is placed still, and the steel pipe is induction heated by the induction heater in the inner wall precise induction quenching device according to any one of claims 1 to 4; The induction heater is pulled by the chain rope to move in the steel pipe, and the induction heater pulls the built-in nozzle to move in the steel pipe together with the chain rope; The internal nozzle sprays cooling water on the inner wall surface of the steel pipe, and the external nozzle sprays cooling water on the outer wall surface of the steel pipe, so that the steel pipe is kept within the set temperature and meets the quenching hardness. The power P of the induction heater is between 600 and 1000 kW, the frequency f is between 1000 and 10000 Hz, the quenching temperature Tq within the range of 1 / 4 to 1 / 2 thickness of the inner wall of the steel pipe is between 950 and 1050° C., and the quenching temperature Te of the outer wall of the steel pipe is between 200 and 600° C.; The quenching hardness within the range of 1 / 4 to 1 / 2 thickness of the inner wall of the steel pipe is between 30 and 50 HRC, and the water flow rate L1 of the built-in nozzle is between 50 and 80 m 3 / h, the water flow rate L2 of the external nozzle is between 30 and 60m 3 / h; The cooling water pressure of the internal nozzle and the external nozzle is between 0.4 and 0.8 MPa.

Images