Process specification of heads for pressure vessels

Scope

This specification is applicable to the forming of heads for pressure vessels such as carbon steel, low alloy steel, stainless steel and stainless steel composite plate.

This specification is a general process technical requirement. The head pressing shall comply with various provisions of this specification. In case of conflict with the design documents and special process documents, the requirements of the design documents and special process documents shall be implemented.

Blank preparation

Blanking of head blank shall comply with relevant provisions of blanking process specification for pressure vessels (SHS/Q-TS 3001-2009).
For the integrally stretched head, the blank thickness is generally 2mm thicker than the design nominal thickness of the head.
After the blank is rounded, the peripheral defects affecting the forming quality of the head shall be polished and eliminated.

Splicing

Groove surface requirements

  • (1) the groove surface shall be free of crack, delamination, inclusion and other defects.
  • (2) The standard tensile strength of steel plate with lower limit σ b >540MPa, and the groove surface of Cr-Mo low alloy steel plate cut by flame, the grinding wheel is applied to grind smooth, and the machined surface is tested by magnetic powder or penetrant.
  • (3) before welding, the oxide, oil stain, slag, dust, iron powder and other harmful impurities within 20mm (calculated from the edge of the groove) on both sides of the groove and its base metal shall be removed.

The butt joint misalignment of the splicing plate shall not be greater than the steel thickness δn 10% of N and not about 1.5mm. The butt misalignment of spliced composite steel plate shall not be greater than 30% of the thickness of steel plate cladding, and shall not be about 1.0mm.
Select qualified welding procedure specification for butt welding.
The weld reinforcement shall be polished to be flush with the base metal before forming the butt weld on the inner surface of the head and the butt weld on the outer surface affecting the forming quality.
The surface of spliced and welded joints shall be free of cracks, undercuts, pores, craters and splashes.
Installation, commissioning and preparation of tooling
Select and design the tire mold according to the head specification.

Determination of design parameters of cold and hot stamping head mould

When hot pressing the head, the shrinkage of the head during hot pressing and the shrinkage rate of the whole head shall be considered δ 3.5/1000 ~ 8/1000.
Shrinkage σ=aΔT100
a: Coefficient of linear expansion
Δ T: Difference between final pressing temperature and room temperature
The springback of cold pressed head shall be considered, and the springback of cold pressed integral head is 3/1000 ~ 7/1000.
Cold forming of carbon steel head shall be avoided. If cold forming is adopted, heat treatment shall be carried out after forming. Austenitic stainless steel heads are preferably cold formed. When hot forming is used, solution heat treatment is required.
Gap Z of hot pressed head = (0.1 ~ 0.2) s, and its value is shown in Table 1.

Table 1 clearance of hot pressed head unit: mm

Plate thickness

6~10

12~14

16~20

22~25

26~30

32~36

38~40

Clearance

0.52.0

1.0~3.0

1.5~4.0

2.0~5.0

2.57.0

3.0~9.0

4.0~12.0

Determination of lower die fillet:
The fillet at the inlet of the lower die is closely related to the stamping quality of the head, generally r = (2 ~ 3) s, and its value is shown in Table 2.
Table 2 Lower die fillet reference table Unit: mm

Plate thickness

6~8

10

12~14

16~18

20~26

28~32

34~36

38~40

Fillet R

20

25

30

40

60

80

100

120

Blank holder measures must be taken during the pressing of hot pressed heads that meet the following conditions.

(1) Elliptical head

  • When DN≤ Φ 1200mm Dw-DN≥20 δn;
  • DN= Φ1200~Φ2000 Dw-DN≥19 δn;
  • DN= Φ2000~Φ3200 DW-DN≥18 δn;

(2) Spherical head

  • DW-DN≥16 δn

(3) Flat head

  • DW-DN≥22 δn

Where: DN – inner diameter of head, DW – expanded diameter,  δn – plate thickness
The drawing die used shall be intact, and the exhaust hole of the upper die shall not be blocked. For the thin-walled head with tensile s / dN not greater than 2%, the wear amount of the pull ring diameter shall not be greater than 0.02s + 2S / DN, and the damage depth of the working face shall not be greater than 1mm.
Stretch and try out the equipment on the press, and install the fetal membrane only after it is confirmed to be intact.
Install the selected pull ring in the cleaned die base, and the lower support shall be evenly distributed and aligned, so that the head can be shelled and pulled out smoothly. The upper mold shall be aligned firmly, the gap between the upper and lower molds shall be uniform, and the gap deviation shall not be greater than 1mm.
Before stretching the head, check whether the tire mold is loose, offset and other defects. It can be used only after it is confirmed to be intact. When there are adverse factors affecting the quality and safety of the head, it shall be stopped.
Before pressing the head, the oxide scale, slag and other wastes on the working surface of the tire mold must be removed, and the lubricant must be applied evenly. When cold stretching the head, the working surfaces of the upper and lower dies and blank holder shall be coated with lubricant, but the middle of the upper die and the middle of the blank shall not be coated to avoid sliding. The lubricant shall be prepared according to the following requirements:

  • (1) carbon steel material: 40% graphite powder and 60% engine oil;
  • ⑵ stainless steel material: 70% graphite powder and 30% engine oil;
  • ⑶ ultra low carbon stainless steel: castor oil (or Vaseline) plus engine oil.

The above lubricants can be mixed into paste.

Stamping forming

Cold forming of carbon steel head shall be avoided. If cold forming is adopted, heat treatment shall be carried out after forming. Austenitic stainless steel heads are preferably formed by cold working. When hot forming is adopted, solid solution heat treatment is required. Heat treatment shall be carried out according to corresponding process documents.
Before stamping, the operator must check the corresponding material identification and number on the head blank according to the process card. When there is a problem, the operator must check the corresponding material identification and number on the head blank.

Blank preloading

In order to avoid wrinkling and bulging when stretching the thin-walled head, the head blank can be pre pressed into an arch (convex).
When δn/DN0.6~2%:

  • (1) Oval head embossing R=0.89~0.9DN;
  • (2) Spherical head bulge R≥1/2DN.

For the head with large and thin diameter, the bulge can be pressed for 2 ~ 3 times, and the tire can be selected from shallow to deep, so as to gradually obtain the ideal r value.
The weld of the blank welded by carbon steel and low alloy steel plate shall be polished and leveled, and then pre pressed into arch shape after stress relief heat treatment.
The arched blank after preloading shall be free of any cracks and other impermissible defects. It can be inspected with a magnifying glass. When there is doubt, it can be subject to surface coloring or magnetic particle testing.
When embossing, the large side of the weld groove of the panel shall be downward.
For thick wall head (DwDN8δn) , in order to avoid excessive thickening of straight edge and excessive thinning of wall thickness during stretching, the blank edge can be processed into an inclined plane according to the figure below, and then pressed.

20210726114314 84371 - Process specification of heads for pressure vessels

Billet heating

Before charging the blank, the sundries on the blank must be cleaned, and the material, plate thickness and specification must be found out before charging the blank as required. The blanks shall be placed in the furnace with supporting pads. When multiple blanks are heated at the same time, 50 ~ 100mm supporting pads shall be added between them. The blanks shall not be directly overlapped to prevent over burning or incomplete burning.
The temperature shall be controlled according to different materials, and the heating temperature shall be as uniform as possible. The temperature in the furnace shall be tested and monitored by thermocouple, and the heating temperature shall be selected according to table 3.
Table 3 Heating temperature and holding time of hot forming steel for common pressure vessels

Material

Q235 series

Q245R

Q345R

15MnVR

15MnVNR

14Cr1MoR

15CrMoR

12Cr2Mo1R

0Cr13

0Cr13Al

Austenitic stainless steel

Heating temperature

950~1000

930~950

9801050

950~1000

1000~1150

Final pressure temperature

850

850

850

850

850~900

Holding time

1min/mm

1.5min/mm

1min/mm

1min/mm

1.5min/mm

The starting and final pressing temperature of stamping shall be controlled strictly according to the process requirements.

The heating of austenitic stainless steel shall meet the following requirements:

  • (1) before heating, the oil stain and other attachments on the surface shall be completely removed;
  • (2) during heating, it shall not be in direct contact with flame or solid fuel, and the heating temperature shall be uniform;
  • (3) the neutral or micro oxidizability of furnace atmosphere shall be controlled. The combustion gas used shall be neutral or micro oxidizable. The sulfur content of fuel oil for heating shall be less than 0.3%. It is strictly prohibited to heat with coal or coke.
  • (4) it is not allowed to heat with carbon steel head in the same furnace.
  • (5) the heating times shall not exceed twice.

Stamping forming

Head blank charging

  • (1) convex blank concave surface upward;
  • (2) the large side of the panel weld faces downward;
  • (3) the trimming of the trimming blank is downward;
  • (4) the composite plate of the composite plate blank is upward;
  • (5) the test plate shall be loaded into the furnace at the same time (when required by the process documents).

During hot pressing of stainless steel head, the upper and lower tire molds shall be preheated, and the preheating temperature shall be more than 150 ℃.
After the blank steel plate is discharged from the furnace, it shall be placed on the matrix immediately, and the oxide scale and sundries on the blank shall be cleaned and corrected quickly to ensure that the temperature during stamping is not lower than the final pressing temperature.
Start the hydraulic press until the upper die comes into contact with the plane of the blank steel plate, and then pressurize.
For heads made of carbon steel and low alloy steel, when 6 δ n≤Dw-DN≤45 δ N, it shall be formed by stamping at one time.
For thin wall head (DW - DN ≥ 45) δ n) , in order to avoid bulge and wrinkle, double forming method can be used.

  • For the first stamping, the lower pull ring with a diameter about 200mm smaller than the upper die is used to punch the blank into a dish shape;
  • The second stamping adopts the upper and lower molds matched with the head specifications, and finally the stamping forming worker.

During head pressing, the speed shall be appropriate to the thickness to avoid excessive head thinning or low final pressing temperature.

Demoulding

The lower part of the head shall be padded with buffer to avoid damaging the head.
Use a special snap ring to block the edge of the head. When the upper die is lifted, the head will fall off.
After the head is demoulded, it needs to be cooled below 550 ℃ (dark brown) before lifting to prevent deformation. The test plate (if any) attached to the head shall be placed in the head and cooled at the same time.
The initial pressure temperature and final pressure temperature of hot pressed head shall be measured by thermocouple thermometer or other appropriate methods, and corresponding records shall be made.

Groove processing

The head groove can be formed by machining and thermal cutting, and the groove type shall be in strict accordance with the requirements of the process documents.

Head shaping

The formed head can be cold reshaped, and hot reshaping and repair welding shall be carried out before heat treatment.
The surface of the workpiece shall not be damaged by the hammer, and the surface of the pad shall not be directly damaged by the hammer.
After the weld joint of the head splicing plate is reshaped, it shall be strictly checked whether there are cracks. If cracks are found, it shall be carried out in accordance with the relevant process regulations.

Heat treatment

Unless otherwise specified in the drawings and process documents, the head made of cold-formed austenitic stainless steel may not be subject to heat treatment, and the head made of other materials shall be subject to heat treatment after processing. When the drawing requires that the container has overall heat treatment, the cold pressed head does not need to be subject to separate heat treatment, but can be subject to overall heat treatment with the container.
The steel plate used in the hot rolling state can be used in the processing state after hot processing without heat treatment.
If the final pressing temperature of the steel plate used in the normalizing state can be controlled above the normalizing temperature of the steel, or the hot working process test plate is qualified, the subsequent normalizing treatment may not be carried out. The evaluation method and qualification standard of processing process test plate shall be implemented according to the requirements of relevant material standards and design documents.
For the steel plate used in normalized + tempered state, if it can meet the above requirements for normalized steel during hot processing, it can only be tempered after hot pressing.
The steel used in the modulation state shall be subject to quenching and tempering treatment after hot processing.
The final pressure temperature of austenitic stainless steel shall be controlled above 850 ℃, and it shall be cooled quickly after processing (blast or water spray cooling). If the material requires intergranular corrosion tendency test and fails to meet the requirements of intergranular corrosion test after hot processing, solid solution treatment or stabilization treatment shall be carried out.

Test

Forming inspection

Inspection sample

The shape tolerance of the inner surface of the head shall be checked with a template, which shall meet the following requirements:

  • (1) use a clearance template with chord length equivalent to the inner diameter of the head or chord length equal to 3 / 4di of the inner diameter of the head;
  • (2) limit deviation of linear dimension of template contour curve: according to the provisions of grade m in GB / t1084;
  • (3) during inspection, make the sample plate perpendicular to the surface to be tested and avoid the weld;
  • (4) the sample can be used only after passing the inspection of the quality inspection station.

Wall thickness inspection

  • (1) the measured minimum thickness of formed head shall not be less than the nominal thickness of head minus the negative deviation Ci of steel plate thickness. When the design drawing indicates the minimum thickness after head forming, the wall thickness of head forming shall not be less than the minimum thickness specified in the design document.
  • (2) the thickness of the formed head shall be measured along the four directions of 0 ゜, 90 ゜, 180 ゜, 270 ゜, using ultrasonic thickness gauge and caliper to detect the thickness of the formed head at the necessary detection parts of the thickness. If there are no special provisions in the design documents, the requirements of JB / t4746 shall be followed.
  • (3) for spliced heads, the weld surface shall not be more than 0.5mm lower than the base metal surface.

Overall dimension inspection

The overall dimension of the formed head shall meet the requirements of design documents, process documents and jb4746 standard.
The following parts shall be measured for the overall dimension of the head:

  • (1) straight edge height and inclination of head;
  • (2) shape deviation of inner surface;
  • (3) outer circumference (or inner diameter, according to the requirements of process documents) of head.

Visual inspection

The formed head is not allowed to have cracks, double skin, overburning and other phenomena.
Surface pits, mechanical damage and other defects after forming shall be polished or repaired. Sharp scars and local scars, grooves and other defects on the stainless steel anti-corrosion surface shall be polished, and the slope of the grinding range shall be at least 1:3. The grinding depth shall be no more than 5% of the steel thickness at this position and no more than 2mm, otherwise it shall be repaired by welding. The welding repair shall be carried out according to the corresponding welding process documents.

Nondestructive testing

After the head is formed, all spliced and welded joints of elliptical, dished and autumn crown heads shall be subject to 100% radiographic or ultrasonic testing according to JB4730 by using the methods specified in the design drawing, and the qualification level shall comply with the provisions of the design drawing.
For class A and B welded joints of flanged conical heads designed according to the rules, 100% radiographic or ultrasonic testing shall be carried out according to the relevant provisions of GB150, the methods specified in the design drawing and JB4730, and the qualification level shall comply with the provisions of the design drawing. When local nondestructive testing is adopted, the tested parts must include the welded joints at the transition section.
For class A and B welded joints of folded edge conical head designed according to analysis, 100% radiographic or ultrasonic testing shall be carried out according to JB4730 by using the method specified in the design drawing, in which radiographic testing level II and ultrasonic testing level I are qualified.
If one of the following conditions is met, magnetic particle or penetrant testing shall be carried out according to JB4730 by using the method specified in the design drawing, and the test result is grade I qualified.

  • (1) surfacing surface of head;
  • (2) composite layer welded joint of composite steel plate head;
  • (3) lower limit of standard tensile strength σ b> The flame cut groove surface of the head made of 540Mpa steel plate and Cr Mo low alloy steel plate, as well as the defect grinding or welding repair surface of the head.

Identification

The product number, specification and material shall be written or sprayed on the qualified outer surface of the head, and the steel inspection seal shall be marked on the outer surface about 100mm away from the end face of the head. When the product is not allowed to be stamped with steel seal, it shall be marked with paint.

Provisions for use of head heating equipment

The head heating furnace shall meet the requirements of uniform heating of workpieces, and shall be equipped with qualified automatic temperature measurement and recording device.
For the automatic temperature measuring and recording device used, the thermocouple shall be calibrated regularly.
After the overhaul of the heating furnace, the temperature distribution in the furnace shall be measured under the supervision of the heat treatment engineer, and a measurement report shall be provided, which is required to control the temperature difference at each point within ± 25 ℃.

Configuration of thermocouple during head heating

The number of temperature measuring thermocouples depends on the size of the workpiece. The workpiece with a diameter of less than 1600mm is equipped with one thermocouple, and the number of thermocouples for large workpiece is not less than two. The temperature difference during heating process is required to be within 4m, 120 ℃, and the temperature difference during insulation process is not greater than ± 25 ℃.

Source: China Head Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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