Once the roll is completed, whatever is inside the roll cannot be seen with naked eye. From outside, roll appears to be normal.
There is every possibility that there are minute surface and sub-surface defects, may be due to Raw Material or Heat Treatment. These defects are to be determined by Non-destructive Testing. The methods are prevalent.

1. ULTRA SONIC TEST
2. MAGNETIC PARTICLE TEST
3. DIE PENETRATION TEST.

1.ULTRA SONIC TEST           

Ultra sonic test is used to detect internal crack by pulse echo- contact method/ back reflection technique. Piezo Electric waves are generated by a probe. While contacting object by probe, these waves propagate in the object and get reflected if any phase change takes place. If there is no phase change, then waves gets reflected from the other end of the reject which return from full depth of material. In case of discontinuity waves are reflected from crack.

Representation on screen will tell whether there is a flaw or discontinuity in the item.

Test is conducted as per ASTM A388/A388M.

Rejection criteria is-
a) Loss in back wall echo exceeding 20% of initial echo.
b) Traveling discontinuity more than 5% of initial echo.
c) Any discontinuity/Flaw equal to or greater than 10% of the B.W.E.

With the use of suitable probe internal defects can be detected. Sometimes there is a signal indicating a flaw at surface, but it is not visible with naked eye. In such case acid etching is done to ascertain surface defect.

Surface to be examined is cleaned by alcohol. Then it is etched with 1:3 or 1:4 solution of nitric Acid in Methanol. Overheated area, grinding cracks, bruise marks, and surface cracks become visible after acid etching. The process of etching indicates clearly the defective regions.

Ultrasonic test in conjunction with acid etching is very reliable.
Ultrasonic test can throw ligh on following defects.

1. Internal crack / flaws :- These generally occur where there is a drastic change in cross section and crack propagates along the axial or longitudinal direction.

2. Piping Defects :- These are forging defects and can be along with the axis in the core zone

3. Slag Inclusion :- These occur during casting the ingots for forging.
4. Shrinkage:- These are generally due to less material while casting the ingots.

5. Internal forging laps/Folds :- These are forging defects an are caused due to overlapping.

6. Porosity. :- These are due to trapped gases/ fumes during casting the ingots.

MAGNETIC PARTICLE TEST
This test conducted to detect cracks on surface and sub-surface layer in ferrous material only.

Normally, Iron particle are suspended in spirit/petrol and the solution is spread over the surface of the rolls after magnetizing the surface. Accumulation of iron particles over surface of rolls indicates cracks. Magnetic particle test is conducted as per ASTM SA 275 & SE 709.

Any magnetic particle build up having width more than 1.0 mm and ratio of length to width more than three times will mean defect/crack.

Following defects can be traced by Magnetic Particle Test.

1 Forging Bursts:- These defects are due to temperature of the metal lesser than required.

2 Flakes :- These cracks may occur at the surface of sub layer of the material and are due to rapid cooling of the rolls during heat treatment.

3 Grinding Cracks :- These surface cracks are developed due to improper grinding.

4 Local heating cracks :- These cracks are developed when the rolls are in service where high pressure forces are in application resulting high work hardening.

5 Stress Cracks :- These are developed while material is in use or in service where material is subjected to heavy alternative or fluctuating stresses.

DIE PENETRAINT TEST (I.S. 12889:1989)

This test is conducted for detecting surface cracks in ferrous and non ferrous material by application of developer and penetraint. Any dark hair line will be treated as defect.

Following fine surface defects can be traced by Die Penetraint test.

1 Surface Hardening Cracks:- This type of cracks generally occurs when the hardness of the surface layer is more than the sub layer hardness.

2. Pitting :- These are pin/ pinch type marks and occur at the surface.

3. Grinding Cracks :- These are due to grinding heats are are called heat checks.

4. Local Heating Cracks :- These cracks are developed at the surface when the rolls are in service where high pressure forces are in application resulting in high work hardening

PREVIEW COLD ROLLING MILL ROLLS MANUFACTURED BY DEE TEE INDUSTRIES

TOOLS THAT LAST LONGER : Slitting Line Tooling, Tube / Section Mill Rolls , Tube Cut Off Knives, Cold Rolling Mill Rolls, Leveller Rolls, Fins, Steel Centers, Chipper Knives, Friction Saws, Shear Blades, HSS Saws,

REQUEST FOR QUOTE

OTHER ARTICLES


SUB ZERO TREATMENT (Reduction in Retained Austenite)
TUBE CUT OFF KNIVES
TIGHTER MANUFACTURING TOLERANCE FOR ROTARY
CUTTER, SLITTING KNIFE AND SPACER.
CRYOGENIC TREATMENT
HINTS & TIPS ON THE APPLICATION OF SLITTER TOOLING
GUIDELINES FOR RAW MATERIAL SELECTION FOR SLITTING LINE TOOLING
SHEAR BLADES AND MACHINE KNIVES
PRECAUTION & ORDERING TIPS FOR FRICTION SAWS
FINS AN IMPORTANT COMPONENT OF TUBE MAKING
IMPORTANT FACTORS IN COLD ROLLING MILL ROLLS
DEE TEE FORGES YET AHEAD IN SEARCH OF EXCELLENCE
SLITTER TOOLING OVER ARM SEPARATORS