Defects of Hot-rolled Seamless Steel Pipe in The Production Process

What defects are prone to occur in the production process of a hot-rolled seamless steel pipe? What are the characteristics? What are the causes? The following will give you a detailed answer:

Cracks

1. Main characteristics

The inner and outer surfaces of seamless steel pipes are straight or spiral cracks, some are mesh-shaped and the ends and bottom of the cracks are sharp.

2. Causes

Poor steel quality, subcutaneous pores and subcutaneous inclusions; improper heating of the tube blank; excessive deformation pressure.

Hair lines

1. Main characteristics

On the outer surface of the seamless steel pipe, there are continuous or discontinuous hair-like fine lines, mostly spiral, the spiral direction is opposite to the rotation direction of the punching machine, the pitch is large, and some are close to a straight line.

2. Causes

Poor steel quality, subcutaneous pores and subcutaneous inclusions; the surface of the tube blank is not thoroughly cleaned. For example, when producing seamless carbon steel pipe, if the tube blank is not properly handled, such problems are prone to occur.

Inner fold

1. Main characteristics

A serrated defect on the inner surface of the steel pipe in a straight or spiral shape. For high-alloy steel pipes, this defect is in the form of irregular blocks and is distributed at the inlet. If the inner fold is caused by centering, it is generally half a circle to one circle and is at the head.

2. Causes

The amount of pressure before the head is too large; the head is severely worn; during the piercing process, the metal in the center of the billet is subjected to strong alternating stress, so the inner wall of the rough pipe is prone to inner fold during rolling; the inner fold at the inlet of the high-alloy steel pipe is caused by uneven force on the roller during piercing.

Outer fold

1. Main characteristics

The outer surface of the seamless steel pipe is spirally folded, and the direction of the spiral is opposite to the spiral direction of the rough pipe on the piercing machine, and the pitch is large.

2. Causes

There are defects such as cracks or ears on the surface of the tube blank; the steel quality is poor, there are inclusions or severe looseness.

Rolling folding

1. Main characteristics

The outer surface of the seamless steel pipe shows regular folding defects.

2. Causes

It is mainly caused by the damage of the piercing machine roll or the tube rolling mill roll.

Tie folding

1. Main characteristics

During rolling, the metal enters the gap between the rolls or the tube loses stability, causing it to form twists and turns. After sizing, it presents local convex and concave wrinkles on the surface of the steel pipe.

2. Causes

The outlet diameter of the leveling machine is larger than the hole type of the sizing and reducing machine; incorrect adjustment, roll misalignment, inconsistent center line, etc.; the sizing machine rack is installed upside down.

Straight folding

1. Main characteristics

The inner surface of the seamless steel pipe is symmetrical or single straight line folded and distributed over the entire length or part of the steel pipe. The inner fold of the straight track will affect the fluidity of the medium inside the pipeline, especially in the seamless heat exchanger tube, which may hinder the heat transfer efficiency and reduce the heat exchange effect.

2. Causes

The head is severely worn or stuck with metal; the outer diameter of the rough tube is larger than the width of the groove hole, the inner diameter is too large or the tube wall is too thin; the pressure reduction distribution of the piercing machine and the tube rolling machine is unreasonable; before the second pass of rolling, the 90° flip is not turned over well.

Straight track

1. Main characteristics

The inner and outer surfaces of the steel pipe show linear scratches with a certain width and depth, distributed over the entire length or part of the steel pipe.

2. Causes

The top rod is bent, causing the head to deflect and scratch the inner surface, and it is not completely flattened during leveling; the head is severely worn or stuck with metal.

Delamination

1. Main characteristics

The spiral or block-shaped layered rupture in the wall or inner surface of the seamless steel pipe.

2. Causes

Non-metallic inclusions and shrinkage holes remain in the tube blank; the tube blank is seriously loose; the heating system of the high-alloy steel tube blank is unreasonable.

Scarring, including warping

1. Main characteristics

The inner and outer surfaces of the steel pipe show scarring.

2. Causes

The inclusions mixed in the lubricant form a hard block with the surface iron oxide scale; the rear end of the rough tube has ears, which fall off during the leveling and are pressed on the outer surface of the steel pipe to form scarring; the quality of the plug is poor and fragile, and its debris is pressed into the inner surface; when the high-alloy steel pipe is perforated, the surface of the plug is oxidized or the position of the receiving trough is improper.

Pits (also known as depressions)

1. Main characteristics

Irregular local depressions and marks on the inner surface of the seamless steel pipe.

2. Causes

Due to scarring or other external injuries such as collision; metal and other objects are attached to the surface of the hole, so that the surface of the hole forms a convex block, which is imprinted on the surface of the seamless steel pipe.

Concave surface (also known as bump)

1. Main characteristics

The wall of seamless steel pipe is concave outside and convex inside, but the wall is not damaged.

2. Causes

Bumping caused by impact during lifting; excessive swing after being bitten by the straightener.

Pockmarks (also known as pitted surface)

1. Main characteristics

The inner surface of seamless steel pipe is localized with small pits in the form of dots, and some also appear periodically.

2. Causes

The groove of the rolling mill or sizing mill roll is rough and has small protrusions; the surface of seamless steel pipe is pressed into iron oxide scale or the surface is severely rusted; the surface of seamless steel pipe is over-pickled and pinhole-shaped and pitting corrosion occurs; when using molybdenum alloy plugs to penetrate high-alloy steel pipes, due to the impact of the plug or the preheating temperature is too low, the glass lubricant is not melted and the softening point of the glass lubricant is too high, it is easy to cause pits on the inner wall of the pipe during perforation.

Convex surface (also called bulge)

1. Main characteristics

The outer surface of the seamless steel pipe shows regular bulges of uneven size.

2. Causes

The rollers of the automatic pipe rolling mill have pitting or depressions.

Concave correction

1. Main characteristics

The surface of the seamless steel pipe shows a relatively smooth spiral depression.

2. Causes

The angle of the straightening machine roller is too small; the amount of pressure is too large or the roller is excessively worn to produce sharp edges.

Scratches

1. Main characteristics

The surface of the thick-walled seamless steel pipe shows spiral or linear groove defects, and the bottom of the groove can be seen in most cases.

2. Causes

The scratches caused at the piercing machine and leveling machine are due to the loss of the guide plate and the outlet nozzle, or the adhesion of iron oxide scale, the wear of the head or the rough surface of the front roller; the scratches caused at the tube rolling mill are due to the outer diameter of the rough tube of the piercing machine being larger than the hole width of the working roller of the tube rolling mill, or the center line of the return roller being inconsistent; other mechanical scratches, such as mechanical damage in the roller, cooling bed, straightening and transportation.

Internal spiral

1. Main characteristics

The inner surface of the seamless steel pipe presents spiral concave and convex.

2. Causes

The diameter expansion of the leveling machine is too large; the surface of the roller, the head and other parts are locally worn; the roller diameter is too small or the wear is serious; the mill is poorly adjusted, and the tool selection is unreasonable; the head is bent and violently jumps; the wall thickness of the rough tube is uneven.

Blue line

1. Main characteristics

A longitudinal straight line mark corresponding to the hole opening of the sizing machine appears on the outer surface of the seamless steel pipe.

2. Causes

The rolls are misaligned or worn severely; the groove turning is poor; the size of the material coming from the leveler is too large.

Burrs

1. Main characteristics

The end of the seamless steel pipe has a whole circle or a part of the cutting residue in the circumferential direction.

2. Causes

The blade angle of the pipe cutting machine is not appropriate.

Summary

Various defects may occur in the production process of hot-rolled seamless steel pipes, such as cracks, hair lines, folds, pitting, scratches, etc. The occurrence of these defects is often related to factors such as steel quality, rolling process, and equipment adjustment.

Read more: Seamless Steel Pipe Sizes