Weld Hardfacing Developments for Grinding Elements of Vertical Mills
and Roller Presses for Cement, Mineral and Coal Industries
Dorival TECCO / Welding Alloys Ltd. / United Kingdom
Modern society depends on the use of vertical mills and roller presses for the production
of an essential range of engineering materials and products. From the old animal powered
configurations since antiquity, these mills have developed into sophisticated pieces of
equipment capable of grinding hundreds of tons per h to accurate product specifications.
Within the scope of this presentation, the term "vertical mill" designates the equipment
consisting of a driven table and a number of hydraulically/spring loaded rollers that are
free to rotate. In this class are included all those configurations described by [Brundiek]
and [EPRI CS-5935, 1988]. The term "roller press" in its turn designates the mill
assembly that uses two rollers, one of which rotates without any horizontal displacement
and another, which is loaded and displaces horizontally. For both vertical mills and roller
presses, it is down to the grinding components to generate the mechanical configuration
and apply the forces that will result in a size reduction. Grinding components hereby
designate the table/group of rollers or the two rollers, respectively for vertical mills and
roller presses.
This paper shall concentrate on the simplest wear system corresponding to the 3-body
abrasion system (e.g. see [Hutchings, 1992]), whereby the grinding elements do not
touch. It shall be said that in practice there may exist some degree of gouging, which is
associated with impact, depending on the properties of the solid being ground. Some mill
designs may also involve the occurrence of metal-to-metal contact, e.g. the bowl
mills (i.e. Fuller-Peters or Babcock and Wilcox mills) and the Foster-Wheeler (i.e. Berz)
mills, however these shall not be discussed in this paper.
"White cast irons" have been used for many decades now to manufacture the grinding
components for vertical mills. Industrial experience over many years led to a narrowing
of the choice of white iron types and in the last years there is a defined tendency to
employ the toughest class I type D and class II type B in ASTM A532. Equivalent
classifications may be found in several other national standards around the world.
Commonly designated respectively as "high-Cr" and "Ni-Hard IV", the highest toughness
relative to the other cast iron types is attributed to a discontinuous M7C3 carbide-type
network [Elliott, 1988].
Unfortunately, none of these are considered weldable with arc processes at room
temperature in the strict metallurgical sense because the heat-affected zone is too hard
and brittle, compromising the structural integrity of the component. Nevertheless,
specialised techniques had been developed successfully, allowing the application of a
hardface to reconstitute the original dimensions or improve the wear performance in
service.
The commonest configuration of roller presses, described in this presentation, involves
the use of a high-quality, high-toughness low-alloy steel to withstand the very high cyclic
compressive stresses. In order to provide the required abrasion resistance, these forgings
are normally hardfaced by welding. Welding and hardfacing are therefore incorporated in
the design and manufacture of new rollers. Other welding and hardfacing operations may
be required at a latter stage in the operational life of the component to reconstitute its
shape or repair localised damages.
This document presents some of the evolutions and status of the hardfacing techniques
for grinding elements in vertical mills and fabrication/hardfacing practices for roller
presses.