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Q&T Bolt Metallography Inspection

[Abstract]:This article details core test indicators, standard procedures, typical defects and acceptance criteria to deliver practical professional guidelines for industrial practitioners.

In the fastener industry, quenching and tempering (quenching + high-temperature tempering) serves as the core process to improve the strength, toughness and fatigue performance of bolts. As an essential detection method, metallographic inspection accurately verifies tempering quality and eliminates potential quality risks. The metallographic structure of tempered bolts directly determines key mechanical properties including tensile strength, yield strength and impact toughness. Substandard microstructure may cause brittle fracture and failure under heavy load and vibration conditions, even leading to severe safety accidents, despite qualified dimensional accuracy. For fastener manufacturers, standardized metallographic inspection systems and professional inspection capabilities are critical to stabilizing product quality and enhancing market competitiveness. This paper comprehensively analyzes bolt tempering metallographic inspection from five dimensions: inspection significance, core evaluation indexes, standardized procedures, common defects and optimization solutions.

I. Core Significance of Metallographic Inspection: Microscopic Quality Control

The primary objective of bolt quenching and tempering is to obtain uniform and fine tempered sorbite, which delivers an optimal balance of strength and toughness and acts as the fundamental quality guarantee for high-strength bolts (Grade 8.8 and above). Metallographic inspection evaluates the effectiveness of tempering processes by observing microscopic characteristics such as microstructure morphology, grain size and component distribution. Its core values are reflected in three aspects:

First, verify the validity of tempering processes. It confirms the formation of standard tempered sorbite and identifies structural abnormalities caused by insufficient quenching temperature, inadequate tempering time and other process deviations. Second, eliminate potential performance risks. Micro defects including microcracks, inclusions and coarse grains that degrade fatigue life can be detected before product delivery. Third, support process optimization. Abnormal metallographic results provide reliable basis for adjusting process parameters such as quenching temperature and cooling medium, so as to improve product consistency.

II. Core Inspection Indexes: Qualification Criteria for Microstructure

The metallographic inspection of tempered bolts focuses on four core indexes: microstructure type and proportion, grain size, decarburization layer depth and internal defects. Evaluation criteria vary according to bolt strength grades and materials, complying strictly with national standards such as GB/T 13320 Metallographic Structure Rating Chart and Evaluation Method for Steel Die Forgings.

1. Microstructure Type and Proportion

Tempered sorbite is the target microstructure. For alloy steel bolts such as 42CrMo and 35CrMo, the proportion of tempered sorbite shall reach no less than 90%. Abnormal structures including untempered martensite and bainite will reduce toughness and increase brittleness, while excessive pearlite will lower structural strength, all of which are judged unqualified.

2. Grain Size

Grain size is positively correlated with mechanical properties, and finer grains contribute to better strength and toughness. Standard requirements specify a grain size grade of 5–8 after tempering. Coarse grains (Grade ≤4) will significantly reduce impact toughness and cause fracture under impact loads.

3. Decarburization Layer Depth

Carbon loss during high-temperature heating forms surface decarburization layers, which weaken surface hardness, wear resistance and thread meshing strength. For high-strength bolts (Grade 8.8 and above), the depth of full decarburization layer shall not exceed 1/3 of the thread height, and the total depth of partial decarburization layer shall be less than 1/2 of the thread height. Exceeding the limit is deemed unqualified.

4. Internal Defects

Key inspection items include microcracks, inclusions and pores. Quenching cracks are linear sharp defects induced by excessive cooling speed and stress concentration, which directly disqualify products. The content of non-metallic inclusions (oxides, sulfides, etc.) shall be controlled below Grade 2.5 in accordance with GB/T 10561. Excessive inclusions form stress concentration points and reduce fatigue strength.

III. Standard Metallographic Inspection Procedures: Full-Cycle Precision Control

Standardized operating procedures ensure accurate and repeatable inspection results. The complete workflow covers sampling, specimen preparation, microscopic observation and result judgment with strict specifications for each step.

1. Sampling

Representative sampling positions include bolt shanks and head transition fillets, where structural abnormalities are most likely to occur during tempering. For batch inspection, 3–5 pieces are randomly selected per batch, with 1–2 specimens taken from each piece. Wire cutting is adopted for sampling to avoid thermal and mechanical damage to the original microstructure.

2. Specimen Preparation

Specimen preparation consists of three steps: rough grinding, fine grinding and polishing. Rough grinding removes cutting traces to flatten the specimen surface. Fine grinding is carried out sequentially with 400#, 800#, 1200# and 2000# abrasive papers to eliminate surface scratches. Diamond polishing agents are used for mirror polishing with surface roughness Ra ≤0.05μm. Finally, specimens are cleaned with anhydrous ethanol to remove residual polishing agents.

3. Microscopic Observation

Specimens are etched with 4% nitric acid alcohol solution for 5–10 seconds to display clear microstructure. Multi-magnification observation (100×, 200×, 400×) is adopted: 100× for grain size and macro defect inspection, and 400× for microstructure type and micro defect analysis. Clear metallographic photos are captured for inspection records.

4. Result Judgment

All indexes are evaluated against national standards and technical requirements. Products with fully qualified indexes are deemed acceptable. If any index fails, the sampling range will be expanded for re-inspection. Persistent unqualified results will lead to batch rejection and mandatory process traceability.

IV. Common Metallographic Defects and Process Optimization

Four typical tempering defects detected by metallographic inspection and corresponding optimization schemes are summarized as follows:

1. Untempered Martensite

Characterized by acicular sharp microstructure, caused by insufficient tempering temperature or short holding time. Optimization: Properly increase tempering temperature (e.g., raise 42CrMo bolt tempering temperature from 550℃ to 580℃) and extend holding time by 10–20 minutes according to bolt dimensions.

2. Coarse Grain

Caused by excessive quenching temperature or prolonged heating duration. Optimization: Reduce quenching temperature (e.g., lower alloy steel quenching temperature from 870℃ to 850℃) and shorten holding time to refine grains uniformly.

3. Over-Limit Decarburization Layer

Caused by high oxidizing furnace atmosphere and excessive heating time. Optimization: Adopt controlled atmosphere furnaces or vacuum furnaces to isolate oxygen, shorten heating time and stabilize furnace carbon potential.

4. Quenching Cracks

Linear extended sharp cracks induced by rapid cooling speed and uneven section size. Optimization: Adopt mild quenching media (replace clean water with quenching oil) and optimize bolt structural design to avoid abrupt section changes and stress concentration.

V. Conclusion: Metallographic Inspection as the Final Quality Barrier

Quenching and tempering determines the core mechanical properties of high-strength bolts, while metallographic inspection ensures reliable and consistent tempering quality. Neglecting metallographic inspection may lead to unqualified products entering the market and causing severe operational risks. A standardized metallographic inspection system enables precise quality control and data-driven process optimization, effectively improving production efficiency and product stability.

Fastener practitioners must attach importance to metallographic inspection, master professional evaluation standards and standardized procedures, and integrate microscopic microstructure control into the whole production process. Only in this way can high-quality high-strength bolts meeting high-end manufacturing requirements be produced, promoting the high-quality development of the fastener industry.