Tough repairs - technical advice casebook

A civil engineering company had a large excavator with a number of badly worn digger teeth on its shovel. The teeth were given to a contractor for reclamation. The material was 13%Mn steel and the method of repair was to weld on cast manganese steel tips after flame cutting and grinding to provide a suitable edge preparation.

For many years the contractor had welded on new tips using 18%Cr-8%Ni austenitic stainless electrodes. Use of stainless steel electrodes had been developed because matching electrodes of the 13%Cr-1%C type were unsuitable for use in multipass welds. This was because of severe embrittlement of the lower runs by carbide precipitation when cooling through and reheating into the 450-900°C temperature range. This embrittlement could cause the welded tips to snap off in service.

To save the cost of stainless steel electrodes, the contractor decided to use electrodes of the AWS EFeMn(a) type containing 13%Mn, 4%Ni, 0.7%C. The addition of nickel allows the carbon to be reduced to below 0.8% so that the weld metal has a lower tendency to carbide embrittlement without any loss of tensile strength or work-hardening characteristics. This allows multipass welds to be made without having to use the more expensive stainless steel electrodes.

In this case all went well until the reclaimed digger teeth were put into service and started breaking off, with cracks propagating through the weld metal and the heat affected zones of the welded tips.

Investigation of the problem showed that there had been no control of the interpass temperature during welding so that the cooling rate through the 900-450°C temperature range had been slow enough to cause carbide precipitation and embrittlement in both the weld metal and the HAZ. The lower carbon content of the 4%Ni-Mn electrodes reduces the embrittlement but does not eliminate it if the cooling rate is slow enough. To prevent embrittlement it is necessary to control the interpass temperature of the weld metal to a maximum of 350°C.

To carry out successful welding or surfacing of 13%Mn steel it is useful to consider the characteristics of this material. It has reigned supreme for its resistance to impact since it was first patented by Hadfield over 100 years ago, and has been used for railway points and crossings, stone crushers and a wide variety of hammers.

Weld metal or castings of manganese steel have a fairly soft (200HV) austenitic structure which in service work hardens at the surface to 500HV or more. The hardened surface resists further deformation and has good abrasion resistance.

The manganese-rich austenite can dissolve more than 1% carbon at high temperatures and when cooled rapidly enough the carbon is retained in solution. When cooled more slowly or when reheated to 500°C or higher, carbides are rejected from solution along relatively few crystallographic planes and the concentrations of carbides along these planes and in the grain boundaries cause severe embrittlement.

A more rapidly cooled casting or weld consisting of austenite work hardens rapidly under impact and the deformation of the surface layers causes small amounts of carbide to precipitate in numerous well-dispersed points. These carbides are so fine and well scattered that their tendency to embrittle is only slight, but their power to increase hardness is marked.

13%Mn steel has extremely high resistance both to impact and to the propagation of cracks. Before the surface work hardens there is little resistance to low stress abrasion, which occurs for example when digging sandy soil which readily erodes the surface.

A common procedure after hardfacing with manganese steel is to deposit single beads of hard alloy steel weld metal (hardness 500-600HV) which will withstand low stress abrasion until the manganese steel work hardens.

To summarise, the rules for welding or surfacing manganese steel are as follows:

Remove any work hardened metal by grinding. This is important because work hardened manganese steel is more susceptible to embrittlement than it is in the soft condition. Areas that cannot easily be indented with a centre punch should be ground out.
Hard surface manganese steel or carbon steel parent metal with manganese steel containing 4%Ni (AWS EFeMn(a)). Alternatively, AWS EFeMn(b) containing 1%Mo can be used.
Weld manganese steel to itself or to carbon steels using either of the above electrode types.
Keep the heat input low with a maximum interpass temperature of the weld of 350°C. The temperature of the zone 12mm away from the weld should not be allowed to exceed 250°C.
When hardfacing thick sections of medium carbon steel or low alloy steel, or when welding manganese steel to these materials, preheating may be required to avoid hydrogen cracking of the HAZs of the hardenable materials. In these cases the hardenable steels should be buttered with a layer of 18%Cr-8%Ni stainless weld metal with preheat, if required, before deposition of the manganese steel.
If abrasive conditions as well as impact are present in service the manganese steel weld metal should have an overlay of hard alloy steel (500-600HV).