Control de calidad de las boquillas forjadas

This article provides a summary and brief analysis of the quality issues that arise during the manufacturing process of takeover forgings from the perspective of equipment supervision, especially the analysis of material melting and non-metallic inclusions, including an understanding of standards, interpretation of production processes and manufacturing elements, recognition of domestic new processes and technologies, and re-learning of the professional foundation of materials science, Intended to improve the quality control of nozzle forging manufacturing.

1. Introduction

Pipe forgings usually bear various mechanical loads and may withstand high temperatures and high pressure, so the requirements for their piezas forjadas are also very strict. The quality of the forgings manufacturing directly affects the quality and lifespan of subsequent processing equipment, and its importance is self-evident. They are taking the nozzle forging made of 16MND5 material as an example for discussion.

2. Manufacturing Introduction

2.1 Materials, processes, and quality control points for connecting forgings

The manufacturing processes of nozzle forgings mainly include material melting, casting, forging, tratamiento térmico, machining, etc. Steel smelting must be done in an electric furnace, with Al added for deoxidation and vacuum degassing. The quality control points of forging manufacturing mainly include commencement review, material melting analysis, performance heat treatment, cutting performance test, mechanical property inspection, NDT (ultrasonic inspection, magnetic particle inspection, liquid penetrant inspection), etc. The delivery status of connecting pipe forgings is pre-welding quenching and tempering.

2.2 Acceptance standards and specifications for nozzle forgings

The acceptance standards and specifications for takeover forgings include a manufacturing outline, quality plan, contract documents, technical specifications, drawings, inspection procedures, and manufacturing and inspection standards specified in the design drawings.

3. Supervision quality control of takeover forgings

3.1 Supervision quality control basis

The main basis for supervision quality control includes manufacturing outline, quality plan, contract documents, technical specifications, drawings, inspection procedures, manufacturing processes, operating procedures, quality assurance documents, supervision management documents, nonconformance (NCR) management regulations, etc.; During the manufacturing process, corresponding standard documents must be strictly followed.

3.2 Introduction and analysis of the main manufacturing processes for nozzle forgings (casting, forging, heat treatment, machining)

  • (1) Material melting: The basic process is steelmaking and preparation → electric furnace smelting → refining furnace refining (phosphorus removal, sulfur removal, aluminum deoxidation, etc.) → dual furnace casting until the tundish is killed and vacuum treated → vacuum casting. The melting process includes multiple melting analyses and chemical composition adjustments;
  • (2) Forging: commonly used hydraulic press for free forging;
  • (3) Performance heat treatment: carried out in industrial gas furnaces and electric furnaces, including normalizing, quenching, and tempering processes;
  • (4) Machining: In specialized processing areas, specialized processing equipment, dedicated personnel, and dedicated positions are used for processing;
  • (5) Mechanical performance inspection: usually equipped with specialized inspection and testing instruments and equipment;
  • (6) Nondestructive testing: Conduct item by item according to design requirements.
  • It is pointed out that supervision and control efforts must be strengthened at key process nodes (casting, forging, heat treatment) in manufacturing forgings, especially in monitoring material sources, cutting off the head and bottom of steel ingots, etc.

3.3 Material Analysis

En material of the nozzle forging must have two key points: weldability and hardenability, excellent mechanical properties, room temperature yield strength, ultimate fracture tensile strength, plasticity, toughness, corrosion resistance, and cold brittleness. The performance of nozzle forgings mainly depends on the material’s properties and casting, forging, and heat treatment processes. The material’s performance is the foundation, mainly determined by its chemical composition (see Tables 1 and 2). Due to its welding performance requirements, its carbon content cannot exceed 0.20%. The mechanical properties and hardenability of this type of steel mainly rely on adding a small amount of alloy elements such as Mn, Si, Ni, and Mo to improve. Mn is a basic element for strengthening, with functions of deoxidation and desulfurization. Its content is generally below 1.6%, and excessive content will significantly reduce the plasticity, toughness, and welding performance of steel. Ni and Mo can refine grains and enhance dispersion, while Si and Al have antioxidant and sedative deoxygenation functions. It should be emphasized that with varying sizes and thicknesses of forgings, hardenability becomes particularly important, as it directly affects the mechanical properties of the nozzle forgings and is also the focus of quality analysis. At the same time, it should also be pointed out that S, P, N, H, and O are harmful elements, and their total amount indicators Σ (S+P+N+H+O) is the main indicator for measuring the purity of molten steel. After 2000, Σ (S+P+N+H+O) ≤ 50 × 10-6 is the goal pursued by China’s forging manufacturing. Dehydrogenation, deoxygenation, and purification of S and P elements have always been essential tasks in metal smelting, and their content is also one of the main indicators for determining the quality of materials. In the production of forgings, the mechanical properties of forgings are often improved by adjusting the chemical composition of the material.

  • (1) Carbon equivalent Ceq = C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/14 = 0.55 – 0.65
  • (2) Soldadura reheat crack sensitivity coefficient △ G = 3.3Mo + Cr + 8.1V-2  ≤ -0.1.

Carbon equivalent is an important reference index for evaluating the weldability of steel.
The higher the carbon equivalent, the greater the tendency for cracks and the poorer the weldability of the steel.
Table.1 Chemical composition requirements for Mn Ni Mo alloy steel forgings used for nozzles (with Al-killed deoxygenation and vacuum degassing)

Elemento Ladle analysis (%) Product Analysis (%)
Carbono <0.20 <0.22
Manganeso 1.15-1.55 1.15-1.60
Fósforo <0.012 <0.012
Azufre <0.012 <0.012
Silicio 0.10-0.30 0.10-0.30
Níquel 0.50-0.80 0.50-0.80
Cromo <0.25 <0.25
Molibdeno 0.45-0.55 0.43 0.57
Vanadio <0.01 <0.01
Cobre <0.20 <0.20
Aluminium Better<0.04 <0.04
Drill <0.03 <0.03

Table.2 Manufacturing Outline of Materials for Pipe Forgings (16MND5) Chemical Composition

Grade 16MND5
Elemento Ladle analysis (%) Product Analysis (%)
C <0.20 <0.22
Mn 1.15-1.55 1.15-1.60
P <0.008 <0.008
S <0.005 <0.005
Si 0.10-0.30 0.10-0.30
Ni 0.50-0.80 0.50-0.80
Cr <0.15 <0.15
Mo 0.45-0.55 0.43-0.57
V <0.01 <0.01
Cu <0.08 <0.08
Al <0.04 <0.04
Co <0.03 <0.03
As 0.01 0.01
Zn 0.01 0.01
Sn 0.002 0.002
B 0.0003 0.0003
H 1.5 ppm 0.8 ppm
O,N provide data provide data

3.4 Supervision Quality Process Control

The supervision work is carried out by standard specifications, manufacturing outline, quality plan, contract documents, technical specifications, drawings, inspection procedures, manufacturing processes, operating procedures, supervision management documents, nonconformance (NCR) management regulations, etc., mainly including witness and inspection of W and H points. These inspections are strictly carried out one by one from personnel, machinery, materials, methods, and the environment.

  • (1) The inspection of “personnel” mainly refers to the qualification inspection of special operation personnel, non-destructive testing personnel, physical and chemical analysis, mechanical performance testing personnel, and visual inspection personnel. For example, personnel engaged in nondestructive testing of pressure vessels should obtain corresponding qualifications, and operators for physical and chemical analysis and mechanical performance testing must also have a technical qualification certificate of intermediate level or above for physical and chemical inspection personnel to be allowed to operate.
  • (2) The inspection of “machines” mainly refers to the review of processing equipment, performance testing machines, hydraulic testing pumps, non-destructive testing instruments, etc., to examine the compatibility, compliance, and effectiveness of the equipment. The equipment and process equipment must be compatible with the tasks they undertake. It must comply with the provisions of corresponding standards, such as room temperature tensile testing machines, high-temperature tensile testing machines, UT inspection instruments, etc., and be fully equipped, in normal, good condition, and within the calibration period. The manufacturing unit must establish management regulations for using and maintaining equipment and process equipment, a job responsibility system, establish equipment archives, and conduct regular inspections.
  • (3) ‘Material’ mainly refers to the raw materials of forgings. Raw materials are the foundation and guarantee for taking over forgings. The supervision and inspection of raw materials are carried out through physical identification inspection, and on the other hand, through the review of melting analysis reports, chemical analysis reports, and completion data of materials for judgment and control.
  • (4) The “law” mainly inspects equipment related manufacturing processes, management procedures, inspection procedures, acceptance standards, etc. Review the compliance and effectiveness of manufacturing processes, procedures, inspection procedures, and acceptance standards, such as whether the cut test material after taking over the forging tempering meets the drawings, whether the sampling method meets the specification requirements, and for example, during the commencement inspection, review the manufacturing outline, quality plan, drawings, inspection procedures, manufacturing processes, enterprise qualifications, personnel qualifications, production and manufacturing capabilities, etc.; Whether the technical documents inspected on site are controlled and effective.
  • (5) Environmental inspections are also necessary. The environment is an important factor affecting the quality of forgings. It is not only a requirement for safe and civilized production but also meets functional and technical requirements. For example, in material smelting, the basic process is steel preparation → electric furnace smelting → refining furnace refining (phosphorus removal, sulfur removal, etc.) → double furnace combined casting to tundish vacuum treatment → vacuum casting. The pre-treatment of the steel ladle, refining furnace, and tundish is very strict, and it is necessary to prevent secondary oxidation and protect the tundish from casting. Part of the non-metallic inclusions in the nozzle forging is self-generated, while the other part is external inclusions that enter the steel ingot. Moreover, the harm of external inclusions to the steel ingot is extremely great, indicating that the quality of the environment directly affects the quality of the nozzle forging. For example, mechanical performance testing requires an environment that is warm in winter and cool in summer to ensure that the state of the testing instruments meets the requirements. For example, ultrasonic testing, liquid penetrant testing, and magnetic particle testing (PT, MT, UT testing) all require the measurement of the surface temperature of the workpiece, as these tests have limitations on temperature conditions. The testing temperature for PT is required to be within 10 ℃ -50 ℃; We use multiple methods to coordinate this.
  • (6) The witnessing of points W and H should ensure 100% attendance, and individual R points can also be witnessed according to point W, such as visual inspection and dimensional inspection of forgings.
  • (7) Patrol inspection is an important means and process for ensuring quality control in manufacturing supervision. It connects and fills the gaps and deficiencies between witness points. Many problems are discovered during inspection, such as damage and failure of production drawings, incorrect identification of workpieces, expiration of testing equipment and measuring tools, isolation of scrapped products, and subsequent processing of non-conforming products.

3.5 Supervision Quality Document Control

Supervision quality document control mainly includes inspection reports, completion reports, quality plan reviews, inspection procedures, NCR reviews, production processes, process drawings reviews, and a large amount of work during the review process involves familiarizing oneself with standards, inspection procedures, acceptance standards, etc.
Taking the acceptance criteria as an example, it is now explained that:
Acceptance standard values for Mn Ni Mo alloy steel forgings used for nozzles:
(1) The specified values for mechanical properties are listed in Table 3.
(2) After final machining, liquid penetrant inspection (PT) must be performed on the surface to be welded. Inspect according to the design-specified inspection standards.
Recording conditions and inspection criteria: Any defect with a size equal to or greater than 1mm should be recorded.
Any defect showing the following indications shall be deemed as unqualified:

  • 1) Linear display;
  • 2) Nonlinear display with dimensions exceeding 3mm;
  • 3) Three or more displays arranged in rows with a spacing of less than 3mm;
  • 4) On a rectangular area of 100cm2, there are 5 or more dense traces. The long side of the rectangle is at most 20cm and is located in the area with the most severe trace assessment.

(3) Volume inspection
Internal defect inspection adopts ultrasonic inspection (UT). The inspection time must be carried out after the final machining of the parts, and the parts that cannot be inspected after forming should be inspected as early as possible. The implementation method of ultrasonic inspection shall comply with the regulations.
Table.3 Required values of mechanical properties

Test items Test temperature ° C Performance Specified value
Circumferential (Horizontal) (3) Axial (longitudinal) (3)
Tension Room temperature R0.002 >400MPa
Rm 550/670MPa
A%(5D) >20
350 R0.002t >300MPa
Rm >497MPa
KV impulse 0 Minimum average 56J 72J
Individual minimum value (1)  40J 56J
-20 Minimum average 40J 56J
Individual minimum value (1)  28J 40J
20 Individual minimum value 72J 88J
(1) Acceptance is only allowed when at most one result in each group of three samples is lower than the specified average value.
(2) If neither the forging program nor the test results can accurately indicate the horizontal indication, samples should be taken at the same depth and tested in both horizontal and vertical directions to obtain accurate individual minimum values.
(3) Sampling depth circumferential: 40X80mm; Axial direction: 80X160mm.

The characteristic parameters of the probe are usually as follows:
Direct beam inspection: According to different structures, the probe frequency is 4MHz or 2MHz.
Oblique beam inspection: According to different structures, the probe frequency is 2MHz or 1MHz, and the reflection angle is 45 °.
(4) Magnetic particle inspection
Any defect with a size equal to or greater than 1mm should be recorded.
All defects that present the following magnetic marks must be marked with their location, removed, or repaired:

  • 1) Linear magnetic traces;
  • 2) Nonlinear magnetic traces with dimensions exceeding 3mm;
  • 3) Three or more magnetic traces arranged in rows with a spacing of less than 3mm; Or magnetic traces with a spacing of 3-6mm and a distribution length greater than 15mm. If the distance between two magnetic traces is less than twice the length of the smaller one, then these two magnetic traces are considered as one magnetic trace. The total length of this magnetic trace should be the sum of the lengths of two magnetic traces plus the distance between these two magnetic traces.

3.6 Supervision coordination and information communication work

Coordination and information communication are the basic abilities that equipment supervision personnel must possess, achieving clear responsibilities, timely communication of major events, and no omission in general communication.

3.7 Common quality issues and control measures

There is a significant amount of non-metallic slag inclusion in the material of the nozzle forging, mainly composed of Si2O3. The focus issue is that the mechanical properties of the forging are not qualified, non-destructive testing is not qualified, and the cutting tool does not meet the requirements. Through holding a special meeting to analyze the causes of quality problems and improvement measures, after discussion and analysis, it was found that the exposed slag is mainly aluminum oxide, which is mainly caused by the production process of steelmaking. The steelmaking production process has always adopted mature methods, processes, technologies, and materials, without any abnormalities, but some shortcomings were found:

  • (1) The casting temperature control is unstable, and the range of casting temperature control is generally very narrow. High temperature casting can ensure that the inclusions have enough time to grow and float during the solidification of the steel ingot. Still, it seriously increases the inclusion in the steel due to erosion of refractory materials, suction during casting, and secondary oxidation. Low temperature casting is not conducive to the inclusion of floating. Unstable casting temperature control, on the one hand, generates too much aluminum oxide, and on the other hand, it shortens the inclusion floating time.
  • (2) The casting speed is relatively low, and the casting speed of steel ingots is around 5t per minute. A low casting speed is equivalent to reducing the casting temperature, which shortens the floating time of inclusions during the solidification process of molten steel, thereby increasing the content of inclusions in the steel ingot.
  • (3) The excessive oxidation and secondary oxidation of molten steel are severe. When the crude refining molten steel C ≤ 0.05%, the oxygen content in the steel is very high, which poses certain difficulties for refining deoxygenation; on the one hand, increasing the use of deoxidizers may increase the oxide content in the steel. During casting, the injection flow is prolonged, the exposure time of molten steel is long, and secondary oxidation is severe.
  • (4) The tundish process is short, and the pouring is started early. In the past, ordinary 60t tundish was used for casting steel ingots. In recent years, a new type of 100t tundish has been widely used for casting in production, which can allow sufficient time for inclusions to gather and grow up and is conducive to the accumulation of inclusions in the mold, which greatly improves the purity of molten steel and improves product quality to a certain extent; Although the problematic steel ingots were cast using a new type of 100t tundish, the molten steel in the tundish was opened for casting when it rose by 3/4 of the height. The process of molten steel in the tundish was short, and the sedation time needed to be longer, failing to achieve the goal of fully floating inclusions.
  • (5) The casting sequence needs to be revised, and the sequence of steel ingots with problems is from small ladle to large ladle. The small amount of steel water and heat storage in a small ladle is not conducive to steel sedation and does not achieve the goal of fully floating inclusions.
  • (6) Due to increased production tasks, frequent personnel changes, and insufficient experience of new employees, process control is flexible, such as unstable casting temperature control and low casting speed.
  • (7) The amount of deoxidizer (Al) added needs to be revised.

Improvement measures:

  • (1) Control the amount and timing of aluminum addition and control its oxidation
  • (2) Using a 100t tundish for casting and strictly requiring the cleaning standards and masonry quality of the tundish, increasing the process and increasing the floating time of inclusions.
  • (3) Control the casting temperature and speed, requiring a casting speed greater than 5.5t/min, and increase the floating time of inclusions in the mold;
  • (4) Promote the application of long nozzle protection casting to avoid secondary oxidation of molten steel and reduce the entry of external inclusions.
  • (5) When the rising height of the molten steel in the tundish is above 1500mm, the pouring should be started to appropriately prolong the cooling time of the molten steel in the tundish, achieving the goal of fully floating the inclusions;
  • (6) Change the casting sequence by pouring the steel ingots together, first casting the large refining package (5 #, 6 #) and then casting the small refining package (7 #).
  • (7) Control the carbon content of rough refining molten steel to prevent excessive oxidation and require a C content greater than 0.05%.

In response to the above issues, a special meeting will be held to analyze and discuss and formulate improvement measures. The correctness of the improvement measures will be verified through future production practices. Here are some suggestions:
Firstly, universal quality issues should be given great attention, and specialized research should be conducted. It is necessary to establish a research group for non-metallic inclusions in steelmaking and make great efforts to solve them, especially to reduce adverse human behavior. The problem of non-metallic inclusions in taking over forgings can be solved; after all, there have been successful experiences and precedents in the past; Secondly, one should be good at innovation. As experts have said, in the production of clean steel, Al2O3 should be eliminated as much as possible before the molten steel enters the crystallizer; Thirdly, writing articles on the tundish, such as using a long nozzle between the refining ladle and the tundish to reduce the total amount of N, adding protective casting (using an immersion nozzle) between the tundish and the crystallizer to prevent secondary oxidation, and adding Mg Ca based coating alkaline lining in the tundish to absorb inclusions effectively.

4. Conclusión

While maintaining traditional and mature methods, processes, and technologies, manufacturing plants must also strengthen and improve in many aspects, such as technology, processes, production, quality assurance, equipment updates, quality innovation, and talent cultivation. Our supervision personnel also face many new problems and challenges, such as familiarity and implementation of response standards and procedures, supervision and execution, implementation of supervision concepts, and performance of supervision functions, all of which require our down-to-earth efforts and dedication. We need to clarify the following:

  • (1) While adhering to the principle of conscientiously doing a good job at witness points, we should strengthen inspection and inspection work and ensure that quality tracking is closely followed and implemented item by item.
  • (2) We should focus on coordinating with various departments of the manufacturing plant, timely grasp the equipment manufacturing status, identify critical paths in the equipment manufacturing process, track the progress of critical paths and nodes, identify and avoid risks as soon as possible, and ensure the smooth progress of the project.
  • (3) Details determine success or failure. Only by looking at the smallest details can we timely detect and stop illegal operations, and at the same time, timely and targeted supervision of manufacturing plants to improve the management level and innovation ability, ensuring the quality of equipment manufacturing.
  • (4) Equipment supervision is a big classroom, where there will be rewards if there is effort and gains if there is hard work.

Author: Jia Lin

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