Quality control of small forgings of nuclear reactor pressure vessel

The quality control of small forgings directly affects the overall quality and mainline progress of RPV. Therefore, taking the quality control of CRDM pipe olet of megawatt inverse nuclear reactor pressure vessel as an example, we discuss how to ensure the product quality of small forgings so as to improve the production efficiency.

20220403063422 21689 - Quality control of small forgings of nuclear reactor pressure vessel

Introduction

In pressurized water reactor nuclear power plants, the reactor pressure vessel (RPV) is the core of the main equipment of the nuclear island, it is the only non-replaceable equipment during the service period of the first circuit equipment, which plays a decisive role in the safe, reliable and long-term operation of the nuclear power plant, and is an important part of the safety defense system of the nuclear power plant. The reactor pressure vessel has various characteristics such as long service time, harsh working conditions, large equipment weight, high dimensional accuracy, long manufacturing period and high manufacturing difficulty, etc. Therefore, the technical conditions are extremely strict and are also the focus of attention of the government’s nuclear safety department and the owner, which is related to the safety and stable operation of nuclear power plants. Among the reactor pressure vessel body materials, there are more than 370 small forgings purchased from outside, which are made of many kinds of materials and occupy most of the RPV body materials. The quality control of small forgings directly affects the overall quality and mainline progress of RPV. Therefore, it is of practical significance to control the quality of small forgings in accordance with regulations, standards, technical conditions and drawings.

Purpose of quality control of small forgings

The purpose of quality control of small forgings is to develop a set of methods for guiding welding, flaw detection, machining, inspection, and implementing prior control, thus realizing batch manufacturing and control of small forgings and changing the status quo of long manufacturing cycle and low efficiency of small forgings. The quality control target of small forgings is to implement the whole process of quality control from the acceptance of forgings into the warehouse to the end of hydrostatic test.

Preparation of quality control documents

Small forgings quality control documents consist of quality plan and various quality control forms. The core of quality control of small forgings is batch control. Quality control forms should be prepared for each process of product acceptance and warehousing, forging decomposition and pairing, welding, flaw detection and finishing respectively, which can ensure accurate forging impressions, fast transmission of quality information and improve production efficiency. Quality plan is the premise of quality control, and the work of preparing quality plan is especially prominent and important in the document preparation stage. The quality plan has the following three roles.

  • (1) In the quality plan preparation stage, it can clearly and completely reflect the product manufacturing process and procedures, and provide the owner to review and select the witness point. The basis for the preparation of the quality plan includes standards, technical conditions, drawings, processes and other documents.
  • (2) In the implementation stage of the quality plan, it truly records the implementation of all processes, the personnel involved and the time, and is the programmatic document for all other records.
  • 3) In the archiving stage of quality plan, it is the core of quality proof documents and is an important document for tracing the quality of products. Therefore, these documents need to be fully prepared in the document preparation stage.

Acceptance of small forgings into storage and out of storage

When accepting forgings, the forging completion report must be reviewed strictly against the technical conditions, forging manufacturing outline and procurement drawings. The forging completion report consists of the following parts.

  • (1) CRDM pipe olet flange material is Z2CN19-10 (nitrogen control), should comply with the provisions of RCC-MM3301 “austenitic stainless steel forgings and stampings for 1, 2, 3 level equipment”.
  • (2) CRDM pipe olet feedthrough material NC30Fe, should comply with the provisions of RCC-MM4108 “hot extrusion NC30Fe nickel-chromium-iron alloy tube”.
  • 3) Chemical composition for melting analysis and finished product analysis.
  • 4) The removal of the head and tail of the ingot, forging ratio.
  • 5) Heat treatment reports and heat treatment curves.
  • 6) Mechanical properties.
  • 7) Metallographic, grain size, non-metallic inclusions.
  • 8) Intergranular corrosion.
  • 9) Results of liquid penetration and ultrasonic flaw detection.
  • 10) Final delivery dimensions.

The core of forgings out of the warehouse is to ensure accurate imprinting and improve the efficiency of the warehouse. The previous practice of using an entry slip to release forgings was inefficient and incomplete in terms of quality information for forgings with large batches like CRDM pipe holders. In order to achieve batch control, all the original information of all CRDM seat forgings are listed and compiled into “CRDM seat forging original information sheet” before the forgings are shipped out, which is convenient for confirming the forging marks and beneficial for the logistics system to order the pieces out of the warehouse.

CRDM pipe olet decomposition and matching

The original forging bar of CRDM pipe olet is a combined forging, each forging bar can be decomposed into multiple products, the decomposition and matching of forging bar is done in the outsourcing factory. It is a key part of the manufacturing process to provide guidance to the outsourcing plant and extend the quality control method to the supplier, which can ensure the quality information can be quickly and accurately transmitted after the product is returned to the plant. The previous practice was to issue a single-piece certificate after the original forging was decomposed, and then pass the single-piece certificate to the outsourcing plant for imprinting, which was not only inefficient but also prone to confusion in imprinting. In order to ensure that there is no error in CRDM tube holder imprinting, before forging bar decomposition, a “CRDM tube holder certificate decomposition list” (see Table 2) is prepared according to the size of the original forging bar, combined with the drawing and decomposition process. Based on this, the CRDM seat flanges and penetrations are matched and compiled into the “CRDM Seat Matching Table”. These two control forms can not only guide the outsourcing factory for decomposition and pairing, but also guide the inspectors, and the speed of delivery inspection is greatly improved. It has been proved that it takes about 2 months for outsourcing decomposition and matching before using the control form, but only 25 days after using the control form. Workers and inspectors with the form can confirm the marks at any time, and there is no longer the phenomenon of matching errors.

Welding and flaw detection quality control

Prerequisites for welding

When inspecting the welding of CRDM pipe holders, in addition to the relevant provisions of RCC-MS7000, the requirements of welding technical conditions should be met. Welding process procedures should be prepared before welding, as the basis for product welding. After welding, a welding data sheet should be prepared to record the process and data of the welding process. Welding process procedures should be consistent with the welding process evaluation report, especially the range of parameters allowed to change. Welding should be implemented by trained, skilled and experienced welders, and the qualifications of welders should be in accordance with the relevant provisions of RCC-MS4000 or HAF603. The qualification of the nondestructive inspector shall conform to the relevant provisions of RCC-MMC8000 or HAF602.

CRDM pipe olet butt welding

CRDM pipe olet is made of CRDM pipe olet penetration and CRDM pipe olet flange group welding. The butt joint shall be welded by automatic TIG with nickel-based wire, and the welding bevel shall conform to the drawing requirements and be consistent with the welding process evaluation. The bevel is qualified by visual inspection (VT) and liquid penetration test (PT), and the surface is clean and free of water, oil and oxides before welding. After assembly, the bevel misalignment should be no more than 1.5mm. attention should be paid to the protection of the molten pool during welding, the temperature between channels should be no more than 225℃, and the oxide of each layer and channel must be cleaned up. After welding, VT and PT inspection should be carried out, but no stress relief heat treatment. after the final machining of the CRDM pipe olet butt weld, the weld should be ultrasonic inspection (UT), radiographic inspection (RT), liquid penetration test (PT), and after the single pipe hydrostatic test, the weld should be tested by UT and PT respectively.

Sealing welding of CRDM pipe olet

Seal welding includes isolation layer and fillet weld. The isolation layer is pre-stacked manually with nickel-based welding rod in the horseshoe nest of the upper head, and the bevel size after finishing should meet the requirements of the drawings and be qualified by visual inspection (VT) and liquid penetration test (PT), and the surface is clean and free from water, oil and oxides before welding. Pre-pile isolation layer before the base material should be preheated, preheating temperature ≥ 150 ℃, inter-road temperature ≤ 225 ℃. Immediately after welding post-heat treatment, the temperature range should be controlled at 250 ~ 400 ℃, at least 2 h. After welding stress relief heat treatment temperature of 615 ℃ ± 15 ℃, holding time of at least 1 h. Angle weld using nickel-based electrode manual welding, and the use of nickel-based wire manual TIG cover, can improve the surface quality of the weld. Welding fillet weld interpass temperature ≤ 225 ℃, and the first layer of the weld and every three subsequent layers of PT inspection. In order to ensure the quality of the weld seam, the arc starting and closing points between the isolation layer and the fillet weld channel must be staggered and need to be polished layer by layer, especially the arc starting and closing points must be cleaned. The appearance and dimensional inspection after welding shall meet the requirements of RCC-MS7461 and product drawings. The final surface of the weld shall be subjected to a PT inspection each after final machining of the top cover assembly and after hydrostatic testing of the reactor pressure vessel. When controlling the welding of CRDM pipe olet, prepare “Welding Process Control Table”, according to which the flange and penetration marks and the CRDM pipe olet marks after group welding can be checked at any time.

Quality control of dimensional inspection

The dimensional inspection of CRDM pipe olet includes single piece dimensional inspection after finishing and dimensional inspection after assembly and welding.The quantity of CRDM pipe olet is large and the dimensional inspection workload is large. Before the dimensional inspection, the dimensional inspection procedure should be prepared according to the drawing and technical conditions so that the inspector can be instructed in detail. mm ~ 0.013mm, surface roughness of Ra1.6μm, and strict through stop gauge inspection requirements. Each CRDM pipe olet is inspected after finishing with many data, and the previous handwritten data is easy to be wrong and not standardized. The dimensional inspection procedure adopts a combination of sketch and spreadsheet, which standardizes the inspection behavior and improves the inspection efficiency. the outer diameter of CRDM pipe olet and upper head interference fit, the interference amount is 0.08~0.10mm. in order to ensure that the upper head hole and CRDM pipe olet are processed simultaneously, all the rest of CRDM pipe olet should be processed first, and when the upper head hole is processed, according to the size of the upper head hole Fine grinding CRDM pipe olet outer diameter, can ensure that the requirements of the drawing in the interference can be met.

Conclusion

It is proved that after adopting the method of prior control and batch control for small forgings and implementing the whole process quality control, the product quality is guaranteed, the efficiency of quality control work is greatly improved, and the generation of NCR can be effectively prevented.

Author: Gong Cheng

Source: Network Arrangement – China Forging Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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