GB/T 38770-2020 in English
VALIDSpheroidite examination and grading for low and medium carbon steels
- Issued on:2020-04-28
- Implemented on:2020-11-01
- File Format:PDF
- Delivery:Within 1 day
$171.00
《GB/T 38770-2020低、中碳钢球化组织检验及评级》由TC75（全国热处理标准化技术委员会）归口，主管部门为国家标准化管理委员会。
Introduction
Analysis of the core content of the standard
This standard has established a complete inspection and evaluation system for the spheroidizing annealing structure of low-carbon carbon structural steel, low-carbon alloy structural steel, medium-carbon carbon structural steel and medium-carbon alloy structural steel. The key technical indicators include:
| Test items | Method requirements | Precision control |
|---|---|---|
| Sample preparation | 3%-5% nitric acid ethanol solution etching | Effective detection area ≥Φ75mm field of view |
| Spheroidization rate calculation | ∑A1/(∑A1+∑A2+0.22∑A3)×100% | Percentage unit precision |
| Magnification | Benchmark 500 times | 1000 times auxiliary observation allowed |
Technical comparison of grading systems
The standard innovatively establishes three sets of independent rating charts, the main differences are reflected in:
| Steel type | Key grading nodes | Organization morphology characteristics |
|---|---|---|
| Low carbon steel | 75% spheroidization rate boundary | Significant increase in point-like spheroidized bodies |
| Medium carbon carbon steel | 80% spheroidization rate boundary | Less residual pearlite |
| Medium carbon alloy steel | 55%/75% double threshold | Higher requirement for carbide distribution uniformity |
Typical application case: ML35 steel for cold heading is required to reach level 4 (＞60%-80%), where spheroidized body and a small amount of pearlite coexist, ensuring both formability and maintaining sufficient strength.
Analysis of Standard Technology Evolution
Compared with traditional inspection methods, this standard has three major breakthroughs:
- For the first time, the 500-fold observation standard of point-like spheroidization is clearly defined
- A 0.22 coefficient is introduced to correct the influence of the area of lamellar pearlite
- A dispute arbitration mechanism is established (based on the calculation method)
Implementation Suggestions
Key points of quality control:
- Sampling should cover the effective heating area of the heat treatment furnace
- The boundary cutting pearlite cluster is calculated according to the actual area
- When observing at 1000 times, it needs to be converted back to 500 times the equivalent area
The inspection report must contain seven core information such as steel grade, furnace number, process status, etc., and comply with the safety and environmental protection requirements of GB/T34895.
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