Question

What kind of defects could occur in:
a. Extrusion
b. Rolling

Answer 1

a) Extrusion defects:

The defects in extruded products can be classified into three categories:

1. Center burst
2. Piping and
3. Surface cracking

1. center burst: Center burst defect is an internal crack that develops as a result of tensile stresses along the center line of the workpart during the extrusion process. Although tensile stresses may not usually be seen in a compression process such as extrusion process, tensile stresses tend to occur under conditions that cause large deformation in the regions of the work away from the central axis. The significant amount of material movement in these outer regions stretches the material along the center of the product. If the stresses are high, a bursting occurs. Conditions that promote center burst defect are high die angles, low extrusion ratios and impurities in the work metal that serve as starting points for crack defects. It is difficult to identify the center burst defect. Center burst is an internal defect that is usually not noticeable by visual inspection. Other names of the center burst defect include Arrowhead fracture, center cracking and Chevron cracking.

2. Piping: Piping is a defect associated with a direct extrusion. As in the above figure (b), it is the formation of a sinkhole at the end of the billet. The use of a dummy block whose diameter is somewhat less than that of the billet helps to avoid a piping defect in the extrusion process. Other names of the piping defect are tailpipe and fishtailing.

3. Surface cracking: Surface cracking defect results from high workpart temperatures that cause cracks to develop at the surface. They often occur when extrusion speed is very high, leading to high strain rates and associated heat generation. Other factors contributing to surface cracking defect in extrusion process are high friction and surface chilling of high-temperature billets in the hot extrusion process.

b) rolling defects:

Defects in rolling can be primarily categorized as

1. General

2. Operational

1)General:

i) surface irregularities: The ingot or the raw material may be having irregularities due to scaling which will get trapped in the metal and remain inside the metal surface as laps. This needs to be removed by grinding and there will be metal loss.If the defect is deep and severe the product may get rejected.

ii) Non-metallic inclusions: The inclusions may results from oxides or nitrides or silicates etc, especially in steels. These are present in the molten metal during the preparation. If less in volume may cause small cracks in the metal and if more in volume will result in severe cracks called crocodile cracks separating the product into two halves.

iii) Internal Pores: There may be pores in the product due to the presence of gases like hydrogen,oxygen,nitrogen etc., If too much gases are present leads to elongation of the pores and the product may become weaker. Sometimes separation may take place resulting in cracks.

2) Operational:
i) waviness-Varying thickness:

Variation in the work across the width in sheet rolling occurs because the roll gap is not perfectly parallel. Since width and volume are constant and thickness is varying, the edges elongate more than the center. But the sheet is a continuous body; the strains readjust to maintain continuity. Thus the center portion is in tension and the edges are in compression. The result is a wavy edge.

ii) Edge Cracking:

The length of the center portion increases but the edges are prevented due to frictional force.As a result the material gets rounded off. The edges are strained in tension leading to edge cracking along the width of the slab. When the difference in the strains become excess i.e. under severe condition, split at the center of the slab occurs.