{"id":1281,"date":"2026-05-25T05:50:43","date_gmt":"2026-05-25T09:50:43","guid":{"rendered":"https:\/\/www.songhaiflange.com\/?p=1281"},"modified":"2026-05-25T05:50:43","modified_gmt":"2026-05-25T09:50:43","slug":"chapter-2-macroscopic-inspection-technology-of-steel","status":"publish","type":"post","link":"https:\/\/www.songhaiflange.com\/es\/2026\/05\/25\/chapter-2-macroscopic-inspection-technology-of-steel\/","title":{"rendered":"Chapter 2: Macroscopic Inspection Technology of Steel"},"content":{"rendered":"<h2>Chapter 2: Macroscopic Inspection Technology of Steel<\/h2>\n<h3>Table of Contents<\/h3>\n<ul>\n<li><a href=\"#section1\">Section 1: Macro-Etch Inspection of Steel (Low-Magnification Acid Etch Inspection)<\/a><\/li>\n<li><a href=\"#section2\">Section 2: Fracture Inspection of Steel<\/a><\/li>\n<\/ul>\n<h3>Preface: Definition and Application of Macroscopic Inspection Technology<\/h3>\n<p>Low-magnification inspection, also known as <strong>macroscopic inspection<\/strong>, is a method for examining the macrostructure and defects of steel and its products using the naked eye or a magnifying glass (under 20\u00d7).<\/p>\n<h4>Advantages:<\/h4>\n<ul>\n<li>Large area, wide field of view, broad coverage<\/li>\n<li>Simple inspection equipment<\/li>\n<li>Faster and more comprehensive reflection of material quality<\/li>\n<\/ul>\n<h4>Applications:<\/h4>\n<p>Used for detecting residual segregation, porosity, inclusions, cracks, and other macroscopic defects in steel products.<\/p>\n<hr>\n<h2 id=\"section1\">Section 1: Macro-Etch Inspection of Steel (Low-Magnification Acid Etch Inspection)<\/h2>\n<h3>Definition of Acid Etch Testing<\/h3>\n<p>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.<\/p>\n<p>It is primarily used to reveal defects such as <strong>cracks, inclusions, segregation, porosity, and flakes (white spots)<\/strong> present in steel.<\/p>\n<p><strong>Reference Standards:<\/strong><\/p>\n<ul>\n<li>National Standard: <strong>GB\/T 226-2015<\/strong> \u2014 &#8220;Acid Etch Inspection Method for Low-Magnification Structure and Defects of Steel&#8221;<\/li>\n<li>ASTM E381 (as applicable)<\/li>\n<\/ul>\n<p>Applicable to round steel, steel billets, steel ingots, and forged steel.<\/p>\n<h3>Sample Selection and Preparation<\/h3>\n<p><strong>General Principles:<\/strong><\/p>\n<ul>\n<li>When inspecting for surface defects (such as cracks), select the outer surface for acid etch testing.<\/li>\n<li>When inspecting steel quality (such as steel ingots), samples should be taken from both ends of the steel material.<\/li>\n<li>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.<\/li>\n<li>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.<\/li>\n<\/ul>\n<h3>Sample Preparation Requirements<\/h3>\n<ul>\n<li>Deformation and work-hardening caused by sampling must be removed.<\/li>\n<li>The test surface should be perpendicular to the direction of steel extension.<\/li>\n<li>Longitudinal sample length is generally 1.5 times the side length or diameter.<\/li>\n<li>For specimens \u226450mm, cold cutting is acceptable; for larger specimens, appropriate cutting methods should be used.<\/li>\n<\/ul>\n<h3>Acid Etch Methods<\/h3>\n<p>Two methods are available: <strong>Cold Acid Etching<\/strong> and <strong>Hot Acid Etching<\/strong><\/p>\n<p>When no special provisions exist in technical conditions, <strong>hot acid etching is the standard method for arbitration testing<\/strong>.<\/p>\n<h3>Acid Solution Formulas<\/h3>\n<table border=\"1\" cellpadding=\"5\" cellspacing=\"0\">\n<tr>\n<th>Steel Type<\/th>\n<th>Etch Time (min)<\/th>\n<th>Acid Solution Formula<\/th>\n<th>Temperature (\u00b0C)<\/th>\n<\/tr>\n<tr>\n<td>Free-cutting steel<\/td>\n<td>5-10<\/td>\n<td>1:1 Industrial hydrochloric acid &#8211; water solution<\/td>\n<td>60-80<\/td>\n<\/tr>\n<tr>\n<td>Carbon structural steel, carbon tool steel, Si-Mn spring steel, ferritic, martensitic, duplex stainless acid-resistant, heat-resistant steel<\/td>\n<td>5-20<\/td>\n<td>1:1 Hydrochloric acid &#8211; water solution<\/td>\n<td>60-80<\/td>\n<\/tr>\n<tr>\n<td>Alloy structural steel, alloy tool steel, bearing steel, high-speed tool steel<\/td>\n<td>15-20<\/td>\n<td>1:1 Hydrochloric acid &#8211; water solution<\/td>\n<td>60-80<\/td>\n<\/tr>\n<tr>\n<td>Austenitic stainless steel, heat-resistant steel<\/td>\n<td>20-40<\/td>\n<td>10 parts hydrochloric acid, 1 part nitric acid, 10 parts water<\/td>\n<td>60-70<\/td>\n<\/tr>\n<tr>\n<td>Carbon structural steel, alloy steel, high-speed tool steel<\/td>\n<td>15-25<\/td>\n<td>38 parts hydrochloric acid, 12 parts sulfuric acid, 50 parts water<\/td>\n<td>60-80<\/td>\n<\/tr>\n<\/table>\n<h3>Defect Rating<\/h3>\n<p>Reference: <strong>GB\/T 1979-2001<\/strong> &#8220;Rating Charts for Structural Steel Low-Magnification Structure&#8221; \u2014 Rating Charts 1-7<\/p>\n<hr>\n<h3>Common Macroscopic Defects in Steel<\/h3>\n<h4>1. Porosity (\u758f\u677e)<\/h4>\n<p>Small voids caused by volume shrinkage during the solidification of steel ingots, or other reasons. Porosity includes general porosity and center porosity.<\/p>\n<p><strong>Causes:<\/strong><\/p>\n<ul>\n<li>Accumulation of impurities and gases in molten steel<\/li>\n<li>Closely related to the cooling rate of steel ingots<\/li>\n<li>General porosity results from faster cooling and dispersed distribution; center porosity results from slower cooling and centralized distribution<\/li>\n<\/ul>\n<h4>2. Segregation (\u504f\u6790)<\/h4>\n<p>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.<\/p>\n<h4>3. Flakes \/ White Spots (\u767d\u70b9)<\/h4>\n<p>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.<\/p>\n<p><strong>Causes:<\/strong> High hydrogen content. After hot working deformation, hydrogen molecules precipitate during cooling, generating enormous internal stress that causes cracking.<\/p>\n<p>Generally, white spots are <strong>unacceptable defects<\/strong> in steel.<\/p>\n<h4>4. Pipe \/ Shrinkage Cavity (\u7f29\u5b54)<\/h4>\n<p>Tubular, trumpet-shaped, or dispersed cavities formed at the center and upper part of the steel ingot due to shrinkage during solidification.<\/p>\n<h4>5. Cracks (\u88c2\u7eb9)<\/h4>\n<p>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.<\/p>\n<hr>\n<h3>Rating Criteria for Common Defects<\/h3>\n<table border=\"1\" cellpadding=\"5\" cellspacing=\"0\">\n<tr>\n<th>Defect Type<\/th>\n<th>Rating Method<\/th>\n<\/tr>\n<tr>\n<td>General Porosity<\/td>\n<td>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.<\/td>\n<\/tr>\n<tr>\n<td>Center Porosity<\/td>\n<td>Based on the number, size, and concentration of dark spots and voids. Rated in 4 grades.<\/td>\n<\/tr>\n<tr>\n<td>Box-type Segregation<\/td>\n<td>Based on the porosity degree of the boxed area and the width of the box band.<\/td>\n<\/tr>\n<tr>\n<td>Spot Segregation<\/td>\n<td>Based on the number, size, and distribution of spots.<\/td>\n<\/tr>\n<tr>\n<td>Center Segregation<\/td>\n<td>Based on the area and number of center dark spots.<\/td>\n<\/tr>\n<\/table>\n<hr>\n<h3>Case Studies<\/h3>\n<h4>Case 4-2-1: General Porosity in GCr15 Steel<\/h4>\n<p><strong>Material:<\/strong> GCr15 Steel<\/p>\n<p><strong>Etchant:<\/strong> 1+1 hydrochloric acid solution, hot etched at 70\u00b0C for 20 min<\/p>\n<p><strong>Condition:<\/strong> Hot-rolled raw material<\/p>\n<p><strong>Observation:<\/strong> Scattered small black dots on the transverse section indicate general porosity. Rated as 2.5 grade per standard \u2014 <strong>unqualified<\/strong>.<\/p>\n<p>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.<\/p>\n<h4>Case 4-2-2: Center Porosity in GCr15 Steel<\/h4>\n<p><strong>Material:<\/strong> 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) &lt;0.020%, w(P) &lt;0.027%]<\/p>\n<p><strong>Etchant:<\/strong> 1+1 hydrochloric acid solution, hot etched at 70\u00b0C for 20 min<\/p>\n<p><strong>Condition:<\/strong> Hot-rolled raw material<\/p>\n<p><strong>Observation:<\/strong> Small black dots at the center of the steel transverse section indicate center porosity. Rated as 1.5 grade per standard.<\/p>\n<p>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.<\/p>\n<h4>Case 4-2-3: Box-Type Segregation in GCr15 Steel<\/h4>\n<p><strong>Material:<\/strong> GCr15 Steel<\/p>\n<p><strong>Etchant:<\/strong> 1+1 hydrochloric acid solution, hot etched at 70\u00b0C for 20 min<\/p>\n<p><strong>Condition:<\/strong> Hot-rolled raw material<\/p>\n<p><strong>Observation:<\/strong> After hot etching, a box-shaped pattern composed of aggregated black dots appears on the steel transverse section, known as <strong>box-type segregation<\/strong>. Rated as greater than 2 grade \u2014 <strong>unqualified<\/strong>.<\/p>\n<p>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.<\/p>\n<h4>Case 4-2-4: White Spots in GCr15 Steel<\/h4>\n<p><strong>Material:<\/strong> GCr15 Steel<\/p>\n<p><strong>Etchant:<\/strong> 1+1 hydrochloric acid solution, hot etched at 70\u00b0C for 15 min<\/p>\n<p><strong>Condition:<\/strong> Hot-rolled raw material<\/p>\n<p><strong>Observation:<\/strong> 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 \u2014 characteristic of white spot defects.<\/p>\n<p>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 &#8220;white spots.&#8221;<\/p>\n<p><strong>Formation cause:<\/strong> Primarily due to high hydrogen content in steel. Under rapid cooling conditions, especially around 200\u00b0C, atomic hydrogen transforms into molecular hydrogen. Hydrogen that has not escaped remains at grain boundaries, generating enormous pressure that causes cracking.<\/p>\n<p>If white spot defects are not exposed to air, they can be addressed through forging. <strong>Generally, white spots are unacceptable defects in steel.<\/strong><\/p>\n<hr>\n<h3>Key Precautions for Acid Etch Testing<\/h3>\n<ul>\n<li>Surface roughness of samples must be ensured; no oil stains or machining marks.<\/li>\n<li>Temperature and time during acid washing must be appropriate.<\/li>\n<li>Corrosion products on the sample surface must be thoroughly brushed clean during rinsing; immediately absorb or blow dry after hot water rinsing.<\/li>\n<li>Samples should be evaluated as soon as possible after acid washing.<\/li>\n<li>For samples that fail acid washing, regrinding must be performed to remove the corroded layer before re-etching.<\/li>\n<\/ul>\n<hr>\n<h2 id=\"section2\">Section 2: Fracture Inspection of Steel<\/h2>\n<p>Fracture inspection can reveal significant problems in steel metallurgical defects and thermal processing.<\/p>\n<p><strong>Reference Standard:<\/strong> GB\/T 1814-1979 &#8220;Steel Fracture Inspection Method&#8221;<\/p>\n<h3>Sample Selection for Fracture Inspection<\/h3>\n<ul>\n<li>When steel diameter or side length is <strong>greater than 40mm<\/strong>, and showing defects such as segregation, non-metallic inclusions, and white spots, <strong>longitudinal fractures<\/strong> should be selected.<\/li>\n<li>When steel diameter or side length is <strong>less than 40mm<\/strong>, <strong>transverse fractures<\/strong> may be used.<\/li>\n<li>Fractures from tensile and impact test specimens after breaking can also be directly used for fracture inspection.<\/li>\n<\/ul>\n<h3>Types of Fractures<\/h3>\n<ul>\n<li><strong>Fibrous Fracture<\/strong> (\u7ea4\u7ef4\u72b6\u65ad\u53e3)<\/li>\n<li><strong>Crystalline Fracture<\/strong> (\u7ed3\u6676\u72b6\u65ad\u53e3)<\/li>\n<li><strong>Laminated Fracture<\/strong> (\u5c42\u72b6\u65ad\u53e3)<\/li>\n<li><strong>Pipe Residual Fracture<\/strong> (\u7f29\u5b54\u6b8b\u4f59\u65ad\u53e3)<\/li>\n<li><strong>Blowhole Fracture<\/strong> (\u6c14\u6ce1\u65ad\u53e3)<\/li>\n<li><strong>Black Brittle Fracture<\/strong> (\u9ed1\u8106\u65ad\u53e3)<\/li>\n<li><strong>Rocky Fracture<\/strong> (\u77f3\u72b6\u65ad\u53e3)<\/li>\n<li><strong>Layered Fracture<\/strong> (\u8418\u72b6\u65ad\u53e3)<\/li>\n<li><strong>Porcelain Fracture<\/strong> (\u74f7\u72b6\u65ad\u53e3)<\/li>\n<li><strong>Platform Fracture<\/strong> (\u53f0\u72b6\u65ad\u53e3)<\/li>\n<li><strong>Foreign Metal Slag Fracture<\/strong> (\u5f02\u91d1\u5c5e\u5939\u6e23\u65ad\u53e3)<\/li>\n<\/ul>\n<hr>\n<p><em>Source: Hangzhou Zhongxiang Mechanical Technology Co., Ltd. \u2014 Technical Training Center<\/em><\/p>\n<h3>Original Document (Chinese Version PDF)<\/h3>\n<p><iframe loading=\"lazy\" src=\"https:\/\/www.songhaiflange.com\/wp-content\/uploads\/2026\/05\/Chapter_2_Macroscopic_Inspection_of_Steel.pdf\" width=\"100%\" height=\"800px\" style=\"border:1px solid #ddd;\"><\/iframe><\/p>\n<p>If the PDF does not display properly, <a href=\"https:\/\/www.songhaiflange.com\/wp-content\/uploads\/2026\/05\/Chapter_2_Macroscopic_Inspection_of_Steel.pdf\" target=\"_blank\">click here to download the original PDF<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Chapter 2: Macroscopic Inspection Technology of Steel Table of Contents 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 [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[227,66,229,230,226,198,228,225,156],"class_list":["post-1281","post","type-post","status-publish","format-standard","hentry","category-blog","tag-acid-etch","tag-flange","tag-fracture-inspection","tag-gcr15-steel","tag-macroscopic-testing","tag-metallography","tag-steel-defects","tag-steel-inspection","tag-steel-quality"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.songhaiflange.com\/es\/wp-json\/wp\/v2\/posts\/1281","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.songhaiflange.com\/es\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.songhaiflange.com\/es\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.songhaiflange.com\/es\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.songhaiflange.com\/es\/wp-json\/wp\/v2\/comments?post=1281"}],"version-history":[{"count":1,"href":"https:\/\/www.songhaiflange.com\/es\/wp-json\/wp\/v2\/posts\/1281\/revisions"}],"predecessor-version":[{"id":1282,"href":"https:\/\/www.songhaiflange.com\/es\/wp-json\/wp\/v2\/posts\/1281\/revisions\/1282"}],"wp:attachment":[{"href":"https:\/\/www.songhaiflange.com\/es\/wp-json\/wp\/v2\/media?parent=1281"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.songhaiflange.com\/es\/wp-json\/wp\/v2\/categories?post=1281"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.songhaiflange.com\/es\/wp-json\/wp\/v2\/tags?post=1281"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}