GB 5168-85

α-β Titanium Alloys — Examination of Structure

UDC 669.295:620.186

Chinese National Standard

Issued: 1985-05-08 | Implemented: 1986-02-01

Approved by: State Bureau of Standards

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Scope

This standard applies to the examination of macro and micro structure of α-β titanium alloys. It is not applicable to finished parts.

1. Macro Structure Examination

1.1 Specimen Preparation

1.1.1 For bars, extruded materials, thick plates, and forging or extrusion billets, specimens shall be cut transversely from the product to be examined, then cut longitudinally in half to examine both transverse and longitudinal surfaces. Specimens from forging or extrusion billets shall be heated to 30±15°C below the β transformation temperature of the tested batch, held for 60±5 minutes, then cooled at a rate equivalent to air cooling or faster. The specimen surface roughness (Ra) shall be 1.78μm (√7) or finer, as determined by GB 1031-83 “Surface Roughness — Parameters and Their Values”.

1.1.2 For forgings, when dimensions permit, the entire outer surface shall be roughly machined to ensure removal of the α-case layer. To eliminate intergranular corrosion and hydrogen embrittlement of finished parts, forged parts after etching shall leave 0.8mm allowance for finish machining. If forgings have been inspected by ultrasonic testing, their surfaces may not require additional machining. The best surface finish shall be 3.3μm Ra (√6) or finer as specified in GB 1031-83.

1.1.3 Specimens shall be free of dust, grease, and other foreign matter, and rinsed with clean tap water.

1.2 Etching

1.2.1 Specimens shall be etched using the following etchant, or other etchants agreed upon by both supplier and purchaser.

1.2.1.1 When using commercially pure acids:

• 13%~17% by volume nitric acid (concentration: 65%~68%)
• 8.5%~11.5% hydrofluoric acid (concentration: 48%)
• Balance: water

1.2.1.2 When using chemically pure acids:

• 13%~17% by volume nitric acid (concentration: 65%~68%)
• 10.5%~13.5% by volume hydrofluoric acid (concentration: 40%~42%)
• Balance: water

1.2.2 The etching rate of the solution should be maintained at 0.05~0.10mm per 5 minutes.

1.2.3 Specimens removed from the etching solution should be immediately rinsed in clean water for several minutes.

1.2.4 Final rinse with pressurized tap water to remove stains, then dry with compressed air.

1.3 Macro Examination

Under illumination of no less than 215 lux, examine specimens with naked eye to check for macro structure and defects, such as cracks, inclusions, and areas where severe defects have not been removed.

2. Micro Structure Examination

2.1 Specimen Preparation

2.1.1 Micro specimens shall be cut from areas deemed necessary on specimens that have already undergone macro examination, or from other surfaces as agreed upon by both parties.

2.1.2 Polish and etch specimens using metallographic techniques to clearly reveal the micro structure to be evaluated.

2.1.3 Mechanical polishing or electrolytic polishing may be used. If electrolytic polishing is used, one of the following three solutions is recommended:

Solution A:

• Methanol: 630ml
• 2-Butoxyethanol: 50ml
• Butyl cellosolve: 260ml
• Acetic acid: 2ml
• Perchloric acid: 60ml
• Voltage: 25~40V, Time: 10~30s

Solution B:

• Distilled water: 783ml
• Ethanol: 700ml (Note: This appears to be an error in the original)
• Perchloric acid: 100ml
• Voltage: 40±1V, Time: approximately 5s

Solution C:

• Perchloric acid: 50ml
• Ethanol (95%): 950ml
• Voltage: 55~60V, Time: 20~40s

2.2 Etching

2.2.1 Recommended etchants, or other etchants agreed upon by both parties:

• 5% hydrofluoric acid (concentration: 40%~42%)
• 12% nitric acid (concentration: 65%~68%)
• Balance: water

2.2.2 Etch specimens with the selected etchant for sufficient time to reveal micro structure. Then rinse in flowing water and dry.

2.3 Specimen Examination

Examine the micro structure of specimens and determine the nature of any suspicious areas found during macro examination.

3. Notes on Structure Evaluation

3.1 The structure evaluation is based on comparison with standard reference images.

3.2 The structure shall be evaluated according to the grade standards specified.

3.3 Segregation can be removed by sandpaper polishing and re-etched to reappear at the original location.

3.4 Segregation resembles other material defects and may appear in products in different sizes, shapes, types, and probabilities.

Appendix A: Safety Precautions (Supplementary)

A.1 Personal Protection

A.1.1 Comply with national regulations on safety and labor protection.

A.1.2 Operators shall be familiar with the properties of chemicals used. Wear appropriate protective clothing during operations.

A.1.3 When preparing etchants, pour acid slowly into water with stirring. Avoid contact between acid and skin.

A.1.4 Due to gas evolution from strong reactions, adequate ventilation must be provided.

A.2 Equipment and Instrument Protection

A.2.1 Containers for macro and micro etchants shall be lined with polyvinyl chloride or equivalent material.

A.2.2 All fixtures, supports, and baskets shall be coated with materials that do not react with nitric-hydrofluoric acid solutions.

A.2.3 Microscope objectives can be corroded by hydrofluoric acid vapor left by etching. When specimens or mounting materials (such as Bakelite) have pores, or when there is leakage between mounting material and specimen edges, acid traces must be carefully removed before placing specimens on the microscope stage. If needed, bake at 80~100°C for 2~3 hours.

Appendix B: Rating Charts (Supplementary)

As needed, select portions of Figures B1~B21 as rating charts for α-β titanium alloy macro and micro structure. Acceptance criteria shall be determined by mutual agreement between supplier and purchaser based on different alloys, processes, specifications, and applications, and specified in product technical conditions.

Figures Reference:

• Figure B1: Macro structure of titanium billet, Grade 10, 1x
• Figure B2: Macro structure of titanium billet, Grade 20, 1x
• Figure B3: Macro structure of titanium billet, Grade 30, 1x
• Figure B4: Macro structure of titanium billet, Grade 40, 1x
• Figure B5: Macro structure of titanium billet, Grade 50, 1x
• Figure B6: Macro structure of titanium billet, Grade 60, 1x
• Figure B7: Macro structure of titanium billet, Grade 70, 1x
• Figure B8: Macro structure of titanium billet, Grade 80, 1x
• Figure B9: Macro structure of titanium billet, Grade 90, 1x
• Figure B10: Macro structure of titanium billet, Grade 100, 1x
• Figure B11: Non-conforming secondary pipe, 1x
• Figure B12: Non-conforming porosity, 1x
• Figure B13: Conditionally non-conforming segregation, 2x (acceptable if micro structure is合格)
• Figure B14: Typical macro structure defects
• Figure B15: Acceptance criteria for macro structure of forgings — qualified, fine macro grains, uniform α-β structure, 1x
• Figure B16: Macro grain size obscured by flow lines, 1x
• Figure B17: Non-conforming coarse macro structure for cross-section below 125mm, 1x
• Figure B18: Non-conforming coarse macro structure for cross-section 125-225mm, 1x
• Figure B19a: Qualified, elongated primary α on transformed β matrix, 100x
• Figure B19b: Qualified, non-deformed β grains without primary α outlines for thick plates with cross-section 180mm and below
• Figure B19c: Qualified, non-deformed β grains without primary α outlines for materials and forgings with cross-section 150mm and below
• Figure B20: Micro structure acceptance criteria, 100x
• Figure B21: Various micro structure examples

Additional Information

This standard was proposed by China Nonferrous Metals Industry Corporation.

This standard was drafted by Beijing General Research Institute of Mining & Metallurgy.

Main drafters: Zhou Guangjue, Tuo Xiangming.

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