Chapter 2: Macroscopic Inspection Technology of Steel

Section 1: Macro-Etch Inspection of Steel (Low-Magnification Acid Etch Inspection)
Section 2: Fracture Inspection of Steel

Preface: Definition and Application of Macroscopic Inspection Technology

Low-magnification inspection, also known as macroscopic inspection, is a method for examining the macrostructure and defects of steel and its products using the naked eye or a magnifying glass (under 20×).

Преимущества:

Large area, wide field of view, broad coverage
Simple inspection equipment
Faster and more comprehensive reflection of material quality

Приложения:

Used for detecting residual segregation, porosity, inclusions, cracks, and other macroscopic defects in steel products.

Section 1: Macro-Etch Inspection of Steel (Low-Magnification Acid Etch Inspection)

Definition of Acid Etch Testing

Acid etch testing utilizes the varying degrees of erosion by acid solutions on different parts of steel materials to reveal the low-magnification structure and defects of steel.

It is primarily used to reveal defects such as cracks, inclusions, segregation, porosity, and flakes (white spots) present in steel.

Reference Standards:

National Standard: GB/T 226-2015 — “Acid Etch Inspection Method for Low-Magnification Structure and Defects of Steel”
ASTM E381 (as applicable)

Applicable to round steel, steel billets, steel ingots, and forged steel.

Sample Selection and Preparation

General Principles:

When inspecting for surface defects (such as cracks), select the outer surface for acid etch testing.
When inspecting steel quality (such as steel ingots), samples should be taken from both ends of the steel material.
When dissecting steel ingots and billets, longitudinal section samples (to show fiber flow lines, band structures, etc.) and two or three transverse section samples (to show white spots, segregation, porosity, subsurface blowholes, etc.) should be prepared.
When performing failure analysis or defect analysis, in addition to sampling at the defect location, a sample should be selected from other representative areas for comparison.

Sample Preparation Requirements

Deformation and work-hardening caused by sampling must be removed.
The test surface should be perpendicular to the direction of steel extension.
Longitudinal sample length is generally 1.5 times the side length or diameter.
For specimens ≤50mm, cold cutting is acceptable; for larger specimens, appropriate cutting methods should be used.

Acid Etch Methods

Two methods are available: Cold Acid Etching and Hot Acid Etching

When no special provisions exist in technical conditions, hot acid etching is the standard method for arbitration testing.

Acid Solution Formulas

Steel Type	Etch Time (min)	Acid Solution Formula	Temperature (°C)
Free-cutting steel	5-10	1:1 Industrial hydrochloric acid – water solution	60-80
Carbon structural steel, carbon tool steel, Si-Mn spring steel, ferritic, martensitic, duplex stainless acid-resistant, heat-resistant steel	5-20	1:1 Hydrochloric acid – water solution	60-80
Alloy structural steel, alloy tool steel, bearing steel, high-speed tool steel	15-20	1:1 Hydrochloric acid – water solution	60-80
Austenitic stainless steel, heat-resistant steel	20-40	10 parts hydrochloric acid, 1 part nitric acid, 10 parts water	60-70
Carbon structural steel, alloy steel, high-speed tool steel	15-25	38 parts hydrochloric acid, 12 parts sulfuric acid, 50 parts water	60-80

Defect Rating

Reference: GB/T 1979-2001 “Rating Charts for Structural Steel Low-Magnification Structure” — Rating Charts 1-7

Common Macroscopic Defects in Steel

1. Porosity (疏松)

Small voids caused by volume shrinkage during the solidification of steel ingots, or other reasons. Porosity includes general porosity and center porosity.

Causes:

Accumulation of impurities and gases in molten steel
Closely related to the cooling rate of steel ingots
General porosity results from faster cooling and dispersed distribution; center porosity results from slower cooling and centralized distribution

2. Segregation (偏析)

Chemical composition non-uniformity formed during the solidification of steel ingots due to significantly different diffusion (movement) speeds of alloying elements (such as carbon, sulfur, phosphorus, etc.) in steel.

3. Flakes / White Spots (白点)

Silver-white spots with smooth surfaces, approximately circular or elliptical in shape, appearing on the longitudinal fracture of steel or forgings containing alloying elements such as nickel, chromium, and molybdenum. After acid pickling, short, discontinuous, generally radially distributed hairline cracks appear in the center of the transverse section and nearby areas.

Causes: High hydrogen content. After hot working deformation, hydrogen molecules precipitate during cooling, generating enormous internal stress that causes cracking.

Generally, white spots are unacceptable defects in steel.

4. Pipe / Shrinkage Cavity (缩孔)

Tubular, trumpet-shaped, or dispersed cavities formed at the center and upper part of the steel ingot due to shrinkage during solidification.

5. Cracks (裂纹)

Cracks exist on or just below the surface of steel materials. After rolling deformation, they exist in the surface or subsurface layer, distributed intermittently along the rolling direction. Some appear as single cracks, while others appear as multiple parallel cracks.

Rating Criteria for Common Defects

Defect Type	Rating Method
General Porosity	Based on the number, size, and distribution of dark spots and voids across the entire section, considering the coarseness of dendrites. Rated in 4 grades.
Center Porosity	Based on the number, size, and concentration of dark spots and voids. Rated in 4 grades.
Box-type Segregation	Based on the porosity degree of the boxed area and the width of the box band.
Spot Segregation	Based on the number, size, and distribution of spots.
Center Segregation	Based on the area and number of center dark spots.

Тематические исследования

Case 4-2-1: General Porosity in GCr15 Steel

Material: GCr15 Steel

Etchant: 1+1 hydrochloric acid solution, hot etched at 70°C for 20 min

Condition: Hot-rolled raw material

Observation: Scattered small black dots on the transverse section indicate general porosity. Rated as 2.5 grade per standard — unqualified.

Porosity is mainly formed when the steel liquid solidifies: volume shrinkage creates interdendritic voids that are not filled by molten steel. This results in a non-dense structure that disrupts the continuity of the steel, reducing mechanical properties. It is prone to cracking and affects the surface roughness of parts after machining, especially for ultra-precision grinding surfaces. Black pits are easily formed, and corrosion can generate rust spots, correspondingly reducing bearing service life.

Case 4-2-2: Center Porosity in GCr15 Steel

Material: GCr15 Steel [w(C) 0.95%~1.05%, w(Mn) 0.20%~0.40%, w(Si) 0.15%~0.35%, w(Cr) 1.30%~1.65%, w(S) <0.020%, w(P) <0.027%]

Etchant: 1+1 hydrochloric acid solution, hot etched at 70°C for 20 min

Condition: Hot-rolled raw material

Observation: Small black dots at the center of the steel transverse section indicate center porosity. Rated as 1.5 grade per standard.

Center porosity is caused by volume shrinkage during metal solidification. Since porosity occurs in the last-to-solidify regions, inclusions tend to concentrate in porous areas. When porosity is severe, it significantly affects the mechanical properties of steel. Porosity can be somewhat improved after hot pressure working.

Case 4-2-3: Box-Type Segregation in GCr15 Steel

Material: GCr15 Steel

Etchant: 1+1 hydrochloric acid solution, hot etched at 70°C for 20 min

Condition: Hot-rolled raw material

Observation: After hot etching, a box-shaped pattern composed of aggregated black dots appears on the steel transverse section, known as box-type segregation. Rated as greater than 2 grade — unqualified.

This defect forms during the solidification of molten steel. According to the general crystallization pattern of steel ingots, the middle portion of the ingot cross-section is the last-to-solidify area, where carbon, alloying elements, and sulfur/phosphorus impurities are more easily enriched, resulting in poor structural density.

Case 4-2-4: White Spots in GCr15 Steel

Material: GCr15 Steel

Etchant: 1+1 hydrochloric acid solution, hot etched at 70°C for 15 min

Condition: Hot-rolled raw material

Observation: After hot etching, numerous fine hairline cracks appear near the center region of the steel transverse section, with serrated edges on both sides of the cracks — characteristic of white spot defects.

Chromium-bearing bearing steel is prone to white spot defects after rolling and air cooling. White spot defects appear as linear cracks on the transverse section of low-magnification test blocks, and as circular or elliptical silvery-white bright crystalline spots on the longitudinal fracture surface, hence the name “white spots.”

Formation cause: Primarily due to high hydrogen content in steel. Under rapid cooling conditions, especially around 200°C, atomic hydrogen transforms into molecular hydrogen. Hydrogen that has not escaped remains at grain boundaries, generating enormous pressure that causes cracking.

If white spot defects are not exposed to air, they can be addressed through forging. Generally, white spots are unacceptable defects in steel.

Key Precautions for Acid Etch Testing

Surface roughness of samples must be ensured; no oil stains or machining marks.
Temperature and time during acid washing must be appropriate.
Corrosion products on the sample surface must be thoroughly brushed clean during rinsing; immediately absorb or blow dry after hot water rinsing.
Samples should be evaluated as soon as possible after acid washing.
For samples that fail acid washing, regrinding must be performed to remove the corroded layer before re-etching.

Section 2: Fracture Inspection of Steel

Fracture inspection can reveal significant problems in steel metallurgical defects and thermal processing.

Reference Standard: GB/T 1814-1979 “Steel Fracture Inspection Method”

Sample Selection for Fracture Inspection

When steel diameter or side length is greater than 40mm, and showing defects such as segregation, non-metallic inclusions, and white spots, longitudinal fractures should be selected.
When steel diameter or side length is less than 40mm, transverse fractures may be used.
Fractures from tensile and impact test specimens after breaking can also be directly used for fracture inspection.

Types of Fractures

Fibrous Fracture (纤维状断口)
Crystalline Fracture (结晶状断口)
Laminated Fracture (层状断口)
Pipe Residual Fracture (缩孔残余断口)
Blowhole Fracture (气泡断口)
Black Brittle Fracture (黑脆断口)
Rocky Fracture (石状断口)
Layered Fracture (萘状断口)
Porcelain Fracture (瓷状断口)
Platform Fracture (台状断口)
Foreign Metal Slag Fracture (异金属夹渣断口)

Source: Hangzhou Zhongxiang Mechanical Technology Co., Ltd. — Technical Training Center

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Chapter 2: Macroscopic Inspection Technology of Steel