Forging standards and comprehensive quality control 

1. Current standard system for forging cores (dominated by domestic standards, with particular emphasis on valves and pressure-bearing forgings) 

Basic universal classification standard 

GB/T 12363-2021 "Classification of Forge Components Functions" 

The forgings are classified into 4 categories based on their risks, which directly determines the strictness of the acceptance process: 

Class I: High-risk pressure-bearing and high-speed rotating components (valve high-pressure bodies, nuclear power forging parts, aircraft rotors), full non-destructive testing, 100% mechanical test samples; 

Class II: Heavy-duty impact components (construction machinery shafts, vehicle steering components), batch sampling flaw detection; 

Class III: Regular static load transmission components, simplified flaw detection; 

Category IV: Secondary supports, non-load-bearing small components, only for appearance size inspection. 

GB/T 8541 Terminology for Forging and Pressing, GB/T 6402-2018 Ultrasonic Testing and Rating of Steel Forgings (Core Criteria for Judging Internal Defects), GB/T 13320-2025 Metallographic Rating of Die Forgings (Judgment of Grain Size, Flow Lines, and Heat Treatment Structure). 

2. Special standards for pressure-bearing / valve forging parts (Required for valve bodies and valve covers of ball valves and gate valves) 

NB/T 47008-2017 "Forgings of Carbon Steel and Alloy Steel for Pressure Equipment" 

The most commonly used industry standards for pipe valves, pressure vessel forgings stipulate forging ratios, heat treatment, mechanical properties, non-destructive testing, and defect limits. These standards serve as the basis for mandatory acceptance of forging products for water supply, steam, and oil/gas valves. 

GB/T 44148.1 - 3 - 2024 "Steel Forgings for Pressure Equipment" 

Compliant with the ISO international pressure-bearing forging standard, it is divided into three categories of alloy forgings: high-temperature, low-temperature, and general-purpose. It is suitable for high-end chemical industry and high-temperature steam valve forgings. 

JB/T 12000-2014 "Technical Specifications for Pressure-bearing Cast Steel Components of Thermal Power Valves" 

Special forged parts for high-temperature and high-pressure power station valves have seen significant improvements in impact toughness and high-temperature strength requirements. 

3. General manufacturing standards for free forging / die forging 

GB/T 37775 General Specifications for Large Free-Forming Forgings 

JB/T 5942 Free forging parts for construction machinery; 

GB/T 37400.8 Heavy-duty Machinery Forgings; 

Material of stainless steel forgings: GB/T 1220; Alloy structural steel: GB/T 3077; Carbon steel: GB/T 699. 

4. Non-destructive Testing Companion Standards (Defect Determination for Forgings) 

UT Ultrasonic Testing: GB/T 6402 (Internal shrinkage cavities, voids, cracks) 

MT magnetic powder: GB/T 15822 (Surface / Near-surface Cracks on Carbon Steel) 

PT Inspection: GB/T 19229 (Surface Opening Defects of Stainless Steel and Austenitic Forgings). 

II. Core mandatory technical requirements for forging standards 

Forging standards (the basic process threshold) 

.1 Forging ratio refers to the degree of deformation during forging. The standard specifies the minimum limit value to ensure a dense internal structure and eliminate the porosity of the steel ingot: 

2. Class I pressure-bearing valve forgings: Axial forging ratio ≥ 3, Radial ≥ 2.5; 

3. Ordinary mechanical free forging parts: ≥ 2; The overall forging ratio of die forging parts ≥ 1.5; 

4. Insufficient forging will result in the presence of porosity and segregation, making it prone to leakage and cracking under pressure. 

2. Chemical Composition Standards 

According to the national standards for material composition, the main elements such as C, Si, Mn, Cr, Ni, Mo are specified, and harmful impurities S and P are strictly controlled. For pressure-bearing forgings, the content of gases H, O, and N is additionally restricted to avoid hydrogen-induced cracks. Minor deviations in the composition of the finished products are allowed, but the fluctuations of key alloy elements must not exceed ±0.05%. 

3. Heat Treatment Standards 

After forging, it must undergo normalizing / quenching and tempering / solution treatment to eliminate forging stresses and homogenize the grain structure. 

2. Carbon steel valve body forgings: Normalizing; 

3. Chromium-molybdenum steel high-temperature valve forgings: Quenched and tempered (quenching + high-temperature tempering); 

4. Stainless steel forgings: Solution treatment; 

5. The mechanical properties, corrosion resistance and crack resistance of the unheat-treated forgings do not meet the standards, and thus they are judged as unqualified. 

III. Comprehensive Quality Requirements for Forgings (Criteria for Complete Acceptance) 

(1) Surface quality of appearance 

The standard prohibits the presence of fatal surface defects: cracks, folds, voids, deep oxidation pits, and through-type sand holes. 

2. For machinable forged parts: Small shallow pits and scratches should be smoothed out with a gentle grinding process without exceeding the machining allowance on either side. 

3. Unprocessed rough forged parts: After defect grinding, the dimensions must not exceed the lower deviation specified in the drawing; 

4. The sealing surfaces and flow channels of the pressure-bearing valve forgings must not be subjected to any grinding, repair or welding. Any minor cracks must be immediately scrapped. 

Bumps, indentations and sharp corners must not cause stress concentration. The length and depth of linear scratches are subject to grade-based restrictions for the forging. 

(2) Dimensions and Positional Accuracy 

Bar stock forgings: Follow the free forging / die forging tolerance standards; The dimensional tolerances of precision valve forgings are tightened by 1 to 2 grades. 

2. The critical wall thickness, the sealing step of the flange, and the inner diameter size of the flow channel are all classified as Class A critical dimensions, and 100% full inspection is conducted. 

3. Bending, eccentricity, and uneven wall thickness deviations will result in local wall thickness being too thin after processing, thereby increasing the risk of pressure explosion. 

(3) Internal metallurgical quality (core of pressure-bearing forgings) 

Based on ultrasonic UT testing, classified according to GB/T 6402: 

2. Not allowed: Internal cracks, through-type shrinkage cavities, dense slag inclusions; 

3. Allowable limit (Class I valve forging): A single point defect equivalent should be no larger than φ3mm, and there should be no continuous or clustered defects. 

4. Low-power acid etching inspection: The flow lines are complete and continuous without any interruptions, white spots, or severe dendritic segregation. The flow lines of the valve body must be distributed along the pressure-bearing contour and it is prohibited for the flow lines to cross the thin-walled areas. 

(4) Mechanical properties (standard mandatory sampling test) 

1. Tensile test: Tensile strength, yield point, elongation after fracture, reduction of area, ensuring plasticity and load-bearing capacity; 

2. Impact: Normal temperature / Low temperature Akv impact energy. Pressure-bearing valves must undergo this test to prevent brittle fracture at low temperatures. 

3. Hardness: The hardness of the material is uniform. The difference in hardness for the same piece does not exceed ±15HBW. If the hardness is too high, it is prone to cracking; if it is too low, the strength will be insufficient. 

4. For Class I forgings, each piece is accompanied by an original sample; Ⅱ Batch extraction of samples in the same furnace with heat treatment, the whole batch of unqualified reinspection, re-heat treatment.