Through continuous attention to customer needs and long-term research and development investment, our new product have recently been put on the market after successful trial in local customers.Today is the second part of this topic.
About the author Dr.Juan Llano:
Deputy general manager of TSR
– Degree in Chemical Engineering. Universidad del País Vasco. Bilbao. 1981.
– Ph.D. course in Metallurgical Engineering. Universidad de Oviedo. 1989.
Part C. Importance of the Roll Material Selection.(RM)
Author:Dr.Juan Llano
In the roughing stands of a bar rolling mill, a roll must respond to two apparently contradictory objectives: They must show a good resistance to wear and, at the same time, present good resistance to the generation of thermal cracks and thermal fatigue. Both properties are contradictory, hence the importance of selecting a good roll material.
Conventional grades used as roll material in the roughing stands of a bar mill train are pearlitic nodular iron with a variable quantity of primary carbides distributed in the working layer and with a hardness between 45 and 60 HSC. This material presents a good balance between wear resistance and thermal fatigue resistance.
On some occasions, when the generation of thermal cracks was very high, a ferritic nodular iron of 40-45 HSC has been proposed in the belief that lower hardness would help reduce the generation of thermal cracks. Big mistake.
Some manufacturers have proposed some alternatives, such as a pearlitic nodular iron with free carbides but refined by a normalizing heat treatment. This treatment refines the structure and adjusts the residual stresses of the roll, slightly improving the resistance to thermal cracks and the performance in the mill.
But we are still talking about a pearlitic nodular iron with free carbides.
As explained above, the generation of thermal cracks is unavoidable when heating / cooling cycles occur which, in turn, generate cycles of tensile / compressive stresses on the roll surface that can exceed the mechanical strength of the material. These can be aggravated or attenuated by the residual stresses in the roll.
The only way to avoid the generation of cracks is to use a material whose mechanical resistance is not exceeded in the traction / compression cycles that initiate the thermal crack or whose resistance and toughness are capable of slowing the propagation of the crack in each cycle once generated on the surface.
The pearlitic nodular iron from a typical roughing roll has a tensile strength in the working layer of about 400-500 MPa.
This value is exceeded in many mills operating at low speed or with poor cooling.
In addition, the presence of iron carbides in its matrix will reduce the mechanical resistance and facilitate the propagation of the thermal cracks once generated on the surface.
Ferritic nodular iron has a tensile strength in the vicinity of 350 MPa, so it does not seem to be the solution and, in fact, it has not been.
The alternatives based on a pearlitic nodular iron with free carbides but refined by a normalizing heat treatment can improve a little the tensile strength and residual stresses and therefore, the performance in the mill.
But we are still talking about a pearlitic nodular iron with free carbides.