GB/T 6402-2008

Steel Forgings — Method for Ultrasonic Testing

Standard Number: GB/T 6402-2008

Replaces: GB/T 68402-1991

เผยแพร่: 2008-05-13 Implemented: 2008-11-01

Issued by: General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China

Adopted from: EN 10228-3:1998 and EN 10228-4:1999

1 Scope

This standard specifies the agreement terms, procedural requirements, personnel qualification, equipment and accessories, calibration and verification, detection timing, surface conditions, sensitivity, scanning methods, classification, reporting levels, and acceptance criteria for ultrasonic testing of steel forgings.

This standard is applicable to manual ultrasonic pulse-echo testing of ferritic-martensitic steel forgings, austenitic and austenitic-ferritic duplex stainless steel forgings. Upon agreement between supplier and purchaser, mechanized scanning methods using immersion testing may also be employed. Forgings from other organizations may also refer to this standard.

Forgings are classified into 4 categories based on shape and production method. Categories 1, 2, and 3 are simple-shape forgings; Category 4 is for complex-shape forgings.

This standard does not apply to: dense die forgings, turbine rotors, and engine crankshafts.

2 Normative References

The following referenced documents are indispensable for the application of this document:

GB/T 9445 — Non-destructive testing — Qualification and certification of NDT personnel (GB/T 9445-2005, ISO 9712:1999, IDT)
GB/T 11343 — Contact ultrasonic angle-beam testing method
GB/T 12604.1 — Non-destructive testing — Terminology for ultrasonic testing
GB/T 18694 — Non-destructive testing — Ultrasonic test probes and their sound field characterization (GB/T 18694-2002, eqv ISO 10375:1997)
GB/T 19799.1 — Non-destructive testing — Ultrasonic testing — Calibration block No.1 (GB/T 19799.1-2005, eqv ISO 2400:1972)
JB/T 4009 — Contact ultrasonic straight-beam testing method

3 Terms and Definitions

Terms and definitions per GB/T 12604.1 apply.

4 Agreement Items

The following items should be agreed upon between supplier and purchaser before testing:

Whether a written procedure submitted for purchaser approval is required
Category and quality grade of the forgings
Whether grid scanning or 100% scanning is required
Whether near-surface zone inspection is required
Additional scanning requirements for Category 4 complex forgings

5 Procedure Preparation

Ultrasonic testing shall be performed according to a procedure. When specified in the contract, the procedure shall be submitted to the purchaser for approval before testing. The procedure shall include at least the following:

Description of the forgings to be tested
Reference documents
Qualification and certification of testing personnel
Production stage at which testing is performed
Testing zones as specified by the applicable quality grade
Couplant used
Description of testing equipment
Calibration and settings
Scanning plan
Description and sequence of test operations
Recording/reporting levels
Discontinuity characterization
Acceptance criteria
Test report format

6 Personnel Qualification

Personnel qualification and certification shall be performed according to GB/T 9445 or applicable standards, and qualified certificates shall be obtained.

7 Equipment and Accessories

7.1 Flaw Detector

Equipment shall use A-scan display and comply with JB/T 10061 requirements.

7.2 Probes

General: Straight-beam and angle-beam probes shall comply with GB/T 18694. Probes with form-fitting shoes may be used when necessary to ensure good, uniform coupling and constant beam angle.

Nominal frequency range: 1.0 MHz – 6.0 MHz

Straight-beam probe: Effective crystal diameter 10 mm – 40 mm

Angle-beam probe: Refraction angle 35° – 70°; effective crystal area 20 mm² – 625 mm²

Dual-crystal probe: May be used if near-surface inspection is required

7.3 Calibration Block

Shall comply with GB/T 19799.1 requirements.

7.4 Reference Block

When sensitivity is set using the Distance-Amplitude Correction (DAC) method, a reference block shall be fabricated. Surface condition of the reference block should represent the surface condition of the tested forging. The reference block shall contain at least three reflectors covering the entire testing depth.

7.5 Couplant

Acceptable couplants include water (with or without anti-rust agents or softeners), glycerin, oil, and water-soluble pastes. The same type of couplant shall be used for calibration, sensitivity setting, scanning, and discontinuity evaluation.

8 Routine Calibration and Checks

Equipment (flaw detector and probes) shall be calibrated and checked according to JB/T 9214.

9 Testing Timing

Ultrasonic testing shall be performed after final heat treatment, unless otherwise agreed at the time of order. For cylindrical and ring forgings about to be drilled, it is recommended to perform ultrasonic testing before drilling.

10 Surface Condition

General: Scanning surfaces shall be free of paint, scale, dried couplant, or any substance that could cause coupling failure, hinder free probe movement, or cause false interpretations.

Surface roughness:

Quality grades 1, 2: Ra ≤ 12.5 μm
Quality grade 3 or higher requirements: Ra ≤ 6.3 μm

11 Sensitivity

11.1 General

Sensitivity shall be sufficient to detect the minimum discontinuity size required for the reporting level of the specified quality grade.

11.2 Straight-Beam Probe

Either of the following methods may be used to determine scanning sensitivity:

DAC method based on flat-bottom holes
Bottom echo reflection method (see Appendix E) — when the scanning surface is parallel to the back surface, or when the cylindrical solid forging thickness/diameter exceeds 3× the probe near-field zone

11.3 Angle-Beam Probe

DAC method using 3 mm diameter side-drilled holes.

11.4 Repeat Testing

If repeat testing is performed, the sensitivity determination method shall be the same as the original.

12 Scanning

12.1 General

Manual contact scanning using pulse-echo method shall be used. The minimum scanning area depends on the forging category. Grid scanning vs. 100% scanning shall be specified in the contract.

12.2 Complex Forgings (Category 4)

For complex-shape forgings or complex sections, supplier and purchaser shall negotiate scanning requirements at the time of order, including: probe angle, scanning direction, scanning coverage (grid or 100%).

12.3 Grid Scanning

Grid scanning shall be performed using one or more probes moved along the grid lines specified in Tables 2 and 3. When a recordable indication is found, additional scanning around the indication shall be performed to determine its extent.

12.4 100% Scanning Coverage

Adjacent probe movements shall overlap by at least 10% of the effective probe diameter.

12.5 Scanning Speed

Manual scanning speed shall not exceed 150 mm/s.

13 Classification

13.1 Forging Categories

Category	Shape	Production Method	Examples
1a	Circular or near-circular cross-section elongated parts	Forged, rolled, or drawn	Profiles, bars, columns, disks, shafts, necks
1b	Square or near-square cross-section elongated parts	Forged, rolled, or drawn	Profiles, bars, blocks, large flat plates cut from ingots
2	Flat, e.g., disks, plates	Forged or cut from plates	Flywheels
3a	Hollow cylindrical	Forged, punched	Rings, pipe blanks, pressure vessel shells
3b	Hollow cylindrical, thin rings/disks	Forged, punched	Ring flanges, thin rings
4	Complex-shape forgings or complex sections	Per manufacturer’s specification	Various complex parts

13.2 Straight-Beam Probe Scanning Coverage (Table 2)

Scanning coverage depends on forging category and dimensions. For 100% scanning, adjacent probe movements shall overlap by at least 10% of the effective probe diameter.

13.3 Angle-Beam Probe Scanning Coverage (Table 3)

For Category 3 forgings: 360° circumferential grid scanning from two directions. Grid interval shall equal radial thickness, maximum 200 mm.

13.4 Discontinuity Classification

Based on dynamic echo patterns from scanning in at least two perpendicular directions:

Point discontinuity: Type I dynamic echo pattern and/or diameter ≤ −6 dB beam width
Linear discontinuity: Type II dynamic echo pattern and/or diameter > −6 dB beam width
Individual discontinuity: Distance between points > 40 mm
Cluster discontinuity: Distance between points ≤ 40 mm

14 Reporting Levels and Acceptance Criteria

The required quality grade shall be negotiated between supplier and purchaser. Four quality grades are defined, with Grade 4 being the strictest (smallest reporting level and acceptance criteria).

Table 4 — Ferritic-Martensitic Steel Forgings

พารามิเตอร์	Grade 1	Grade 2	Grade 3	Grade 4
Nominal Frequency (MHz)	≥2	≥2.5	≥4	≥5
Reporting Level (flat-bottom hole dia.)	>8 mm	>5 mm	>3 mm	>2 mm
Acceptance — Individual point discontinuity	<12 mm	<8 mm	<5 mm	<3 mm
Acceptance — Linear/cluster discontinuities	<20 mm	<15 mm	<10 mm	<6 mm (≥DAC3/3mm)

Table 5 — Austenitic and Duplex Stainless Steel Forgings (Straight-Beam)

Reporting levels and acceptance criteria are based on equivalent flat-bottom hole diameter, specified by thickness range and quality grade (1 through 4). Grade 1: ≥Φ8mm reporting, <5mm acceptance for thickness ≤250mm. Grade 4: ≥Φ2mm reporting, bottom wave attenuation factor (Ff) criteria, complete bottom wave loss ≥5% of initial bottom echo.

Table 6 — Austenitic and Duplex Stainless Steel Forgings (Angle-Beam DAC)

Based on 3 mm diameter side-drilled holes. Recording levels and acceptance criteria specified by forging thickness and nominal frequency. For thickness ≤400 mm.

15 Sizing

For linear discontinuity evaluation, one or more of the following methods shall be used (per JB/T 4009):

−6 dB method
−20 dB method
Peak comparison method

16 Report

The test report shall include the following information:

Name of the purchasing party
Identification of the tested material
Testing scope, testing zones, and quality grade
Timing of ultrasonic testing
สภาพพื้นผิว
Equipment used (flaw detector, probes, calibration and reference blocks)
Sensitivity setting method
Standard or procedure used
Testing results: discontinuity location, classification, and sizing (expressed as flat-bottom hole equivalent diameter or side-drilled hole percentage)
Details of scanning coverage and whether near-surface zone applies
Date of testing
Name, qualification, and signature of the operator

Appendices (Informational)

Appendix A: Clause number comparison with EN 10228-3:1998 and EN 10228-4:1999
Appendix B: Technical differences between this standard and EN standards and their reasons (e.g., DGS method removed and replaced by bottom echo method, which is not commonly used in China)
Appendix C: Maximum detectable depth for circumferential angle-beam scanning (probe angles 35°–70°)
Appendix D: DAC (%) amplitude to dB value correspondence table
Appendix E: Method for sensitivity adjustment using workpiece bottom echo (formulas for solid and hollow forgings)

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