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Common macroscopical defects of steel

In the process of smelting or hot processing, the internal or surface defects of steel are caused by some factors (such as non-metallic inclusions, gas and process of improper operation, etc.), which seriously affect the quality of the material or product, and sometimes it will cause material or product to be scrapped.
The porosity, bubble, shrinkage residual, non-metallic inclusions, segregation, white spots, cracks and various abnormal fracture defects of the steel can be found by macro test. The macroscopic inspection method consists of two methods: acid leaching test and fracture inspection. The common macro defects shown by acid leaching are summarized below.
Segregation
Cause of formation
During the solidification process, some elements are aggregated due to the selection of crystallization and diffusion, resulting in uneven chemical composition. According to the different locations of distribution, they can be divided into ingot shape, center and dot segregation.
Macro characteristics
In the acid leaching sample, when the segregation is an erosive material or gas inclusion, it is dark dark, irregular in shape, slightly concave, flat at the bottom and a lot of dense microporous spots. If the elements are gathered for corrosion resistance, they are slightly colored, irregular and smooth.
Loose
Cause of formation
In the course of solidification, the steel can not be welded in the hot working process due to the final solidification and contraction of the low melting point material and the voids generated by the gas emission. According to its distribution, it can be divided into two categories: Center and general loose.
Macro characteristics
On the transverse hot acid soaking surface, the pores are irregular polygons and dimples at the bottom, which usually occur in segregation spots. In serious cases, there is a tendency to be spongy.
Inclusion
Cause of formation
(1) foreign metal inclusion
During casting, metal strips, blocks and sheets fall into the ingot mold or the iron alloy added at the end of smelting is not melted.
Macro characteristics
On the etched sheet, there are many distinct shapes with distinct edges and different colors.
Cause of formation
(2) Foreign non-metallic inclusions
During casting, there is no time to come out of the slag or the refractory material that is spalling into the lining of the molten steel and the inner wall of the pouring system.
Macro characteristics
The larger non-metallic inclusions are well identified, while smaller inclusions are spalling after corrosion, leaving tiny circular pores.
Cause of formation
(3) The semi solidified film on the surface of the bottom ingot is embedded in the molten steel.
Macro characteristics
On the acid leaching samples, the color is different from the surrounding area, and the irregular curved narrow strip is surrounded by oxide inclusions and pores.
Shrinkage hole
Cause of formation
When the ingot or casting is poured, the liquid in the core can not be replenished due to the volume contraction at the last condensing, and the macro hole is formed in the ingot head or casting.
Macro characteristics
On the transverse acid leaching sample, the shrinkage cavity is located at the central part, which is often surrounded by segregation, inclusion or loose density. Sometimes holes or gaps can be seen before etching. After etching, the holes become darker and appear irregular wrinkled holes.
Bubble
Cause of formation
Defects caused by gas generated and released during ingot casting.
Macro characteristics
On the transverse specimen, there are cracks that are approximately perpendicular to the surface and slightly oxidizing and decarburization nearby. Beneath the surface there is a hypodermic bubble, and a deep subcutaneous bubble is called pinhole. During the forging process, the pores which are not oxidized and not welded are extended into thin tubes, and the cross sections are isolated small pinholes. The cross section is similar to the punctate segregation of the arrangement, but the deeper color is the inner honeycomb bubble.
White spot
Cause of formation
It is generally believed that hydrogen and tissue stress play an important role in the segregation and inclusion of steel.
Macro characteristics
In the transverse hot acid leaching sample, there is a fine short crack. On the vertical fracture surface, it is a coarse crystalline silver white spot.
Crack
Cause of formation
Axial intergranular cracks: when the dendrites are serious, large cracks occur along the main stem and branch of the twigs.
Internal crack: cracking due to improper forging process.
Macro characteristics
On the cross section, the axis position is intergranular crack, which appears spider net, and radially dehiscence is serious.
Fold
Cause of formation
The surface of the steel or ingot is uneven and the edge angle of the tip. It is superimposed on the steel during the forging and rolling, or the ears are produced due to the improper design or operation of the pass.
Macro characteristics
On the transverse hot acid leaching sample, there is a diagonal crack on the surface of the steel, and there is more serious decarburization near it.
Source: China Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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