Alligatoring - 9Cr-1Mo steel

Alligatoring - 9Cr-1Mo steel - Rolling defects

Figure 1: Failure of AR material by alligatoring after about 40% reduction at room temperature rolling.

Figure 2: Micrograph showing cracks associated with carbides.

Figure 3: Multiple cracks at planes away from centre in the sheet failed by alligatoring. These cracks remain stable and do not take part in final failure.

Defect name: Alligatoring
Record No.: 676
Type of defect (Internal/Surface): Internal
Defect classification: Rolling defects
Steel name: 9Cr-1Mo tool steel
Steel composition in weight %: 0.10% C, 0.35% Mn, 0.50% Si, <0.025% P, <0.025% S, 9.50% Cr, 1.00% Mo, 0.08% Nb, 0.21% V, 350 ppm N.
Note: Alligatoring damage was encountered during the rolling of a modified 9Cr-1Mo steel in the as-received condition. The cause of this failure was attributed to the banded nature of the microstructure formed by coarse carbides, which aids the nucleation of the voids at the interface. The banded microstructure was destroyed by a heat treatment which gives a uniform fine distribution of carbides. Such a microstructure could eliminate the alligatoring problem in this steel.

The modified 9Cr-1Mo steel was received as a hot rolled billet with square cross section of 120x120 and 240 mm long, the chemical composition for which is given in the upper text.

The AR plates were rolled in a two high mill (roll diameter=190 mm) at room temperature such that the rolling direction parallel to the transverse direction. Cold rolling was done under heavy lubrication (using lubricating oil) in order to minimise the friction, which is suspected to be possible cause of alligatoring. The rolls were greased after each pass. The plates started failing by alligtoring after a reduction in thickness of about 30%-40% during room temperature rolling (with a reduction 5pct/pass) (Fig. 1). Rolling of rods (~25 mm in diameter) cut along the transverse direction of the billet were also attempted and the material also failed by alligatoring. The 710T plates were rolled under exactly similar conditions at room temperature and the plates could be rolled from a thickness of 20 to 2 mm without alligatoring defect.

The phenomenon of alligatoring in this steel can be attributed to the presence of carbides lying as bands parallel to the sheet rolling direction. The carbides in the centre plane of the plate are under tensile loading during the initial stages of rolling and the voids nucleate at the carbide-matrix interface. The voids subsequently join up to form small cracks. As more voids accumulate, the crack extends in length, in the direction of rolling, with increasing strain (Fig. 1). The stresses in the plate are expected to reverse as the plates become thin. This should cause tensile stresses away from the centre plane and the cracks should form in these planes as well at the carbide± matrix interface. Fig. 3 shows stable cracks in the planes away from the centre plane. The cracks are not expected to `heal' in the course of rolling because of the carbide particles within them. The final failure is further assisted by the concavity developed at the front end of the sheet which can act as stress riser. This acts as the initiation site for the alligatoring failure. The normal stresses that exist at the front end of the plate at the exit from the rolls, is sufficient to spilt the slab along the centre plane, which is already weakened by small cracks. Thus the failure is invariably along the centre plane of the sheet even if cracks are present in other planes as well.
Reference: Not shown in this demonstration version.