“Welding Wear Resistant Steel”
Wear resistant steels are used
extensively in the mining and aggregate industries for the manufacturing and
refurbishment of shoots, buckets, bucket lips, box liners, and other equipment
that sees regular exposure to hard and abrasive materials.
Because of their unique properties, these steels have very specific requirements regarding their machining and welding. Welding procedures, in particular – if not performed appropriately – can cause damage and cracking, or a less obvious softening of the steel, resulting in significant increase in cost and decreased service life of the weldments.
As the reasoning behind the
requirements listed in the WPS and WPDS may not always be obvious at first
glance, the following tries to outline those reasons, and why it is important
to educate everyone in the production process on the proper welding of wear
resistant steel.
A
Background on Wear Resistant Steel
Wear resistant steel comes in both
wrought and cast form. Both wrought and
cast forms are typically steels with sufficient hardenability (see Wikipedia
article here
for a definition) to
allow them to be heat treated to produce a martensite or martensite/bainite
structure that is subsequently tempered to the desired hardness level.
In the heat treatment process, these
steels are typically treated to a desired Brinell hardness level which can
range from as low as 350 up to 500.
The process is very sensitive to
temperature, time, and the alloying of the steel itself. All wear-resistant steels are not the same,
and therefore behave differently when welding and treating. In order to achieve the hardness levels
desired when producing the steel, the chemical composition of these steels is
typically increased with the thickness of plate being produced.
Table 1 presents maximum chemical
compositions for wrought wear resistant steel and Table 2 presents actual data
for a few cast alloys.
Table 1 Chemical
Composition of Wear Resistant Wrought Steels
|
Name
|
C
|
Mn
|
Si
|
Cr
|
Ni
|
Mo
|
V
|
B
|
CEV
typ
|
UTS
(MPa)
|
Thickness
(mm)
|
|
Hardox
|
0.15
|
1.60
|
0.70
|
0.50
|
0.25
|
0.25
|
|
0.004
|
0.42
|
1250
|
8-20
|
|
400
|
0.18
|
1.60
|
0.70
|
1.00
|
0.25
|
0.25
|
|
0.004
|
0.48
|
1250
|
20-32
|
| |
0.22
|
1.60
|
0.70
|
1.40
|
0.50
|
0.60
|
|
0.004
|
0.57
|
1250
|
32-45
|
| |
0.32
|
1.60
|
0.70
|
1.40
|
1.50
|
0.60
|
|
0.004
|
0.73
|
1250
|
80-130
|
|
Dillidur
|
0.20
|
1.60
|
0.50
|
1.00
|
1.5
|
0.70
|
0.10
|
0.004
|
0.43
|
1200
|
10
|
|
400V
|
|
|
|
|
|
|
|
|
0.46
|
1200
|
25
|
| |
|
|
|
|
|
|
|
|
0.51
|
1200
|
40
|
| |
|
|
|
|
|
|
|
|
0.61
|
1200
|
80
|
| |
|
|
|
|
|
|
|
|
0.64
|
1200
|
120
|
|
AlgoTuf
|
0.17
|
1.50
|
0.45
|
0.20
|
|
0.20
|
|
0.003
|
0.37
|
1206
|
5-13
|
|
400F
|
0.17
|
1.50
|
0.45
|
0.25
|
|
0.20
|
|
0.003
|
0.41
|
1206
|
13-20
|
| |
0.20
|
1.50
|
0.45
|
0.70
|
|
0.35
|
|
0.003
|
0.51
|
1206
|
20-25
|
| |
0.26
|
1.50
|
0.45
|
0.60
|
|
0.65
|
|
0.003
|
0.57
|
1206
|
25-70
|
| |
|
|
|
|
|
|
|
|
|
|
|
Compositions
are maximum values in wt%
CEV(IIW) = C + Mn/6
+ (Cr+Mo+V)/5 + (Ni+Cu)/15
Table 2 Chemical
Composition of Wear Resistant Cast Steels
|
Name
|
C
|
Mn
|
Si
|
Cr
|
Ni
|
Mo
|
V
|
B
|
CEV
(act)
|
Hardness
(HB)
|
Thickness
(mm)
|
| |
|
|
|
|
|
|
|
|
|
|
|
|
A
|
0.198
|
1.01
|
0.50
|
1.04
|
1.95
|
0.25
|
0.02
|
|
0.67
|
400
|
50
|
| |
|
|
|
|
|
|
|
|
|
|
|
|
B
|
0.260
|
0.75
|
0.30
|
0.89
|
1.95
|
0.23
|
0.01
|
|
0.79
|
400
|
50
|
| |
|
|
|
|
|
|
|
|
|
|
|
|
C
|
0.285
|
0.94
|
0.39
|
0.286
|
0.514
|
0.188
|
|
|
0.56
|
400
|
25
|
| |
|
|
|
|
|
|
|
|
|
|
|
|
D
|
0.287
|
1.54
|
2.35
|
0.66
|
0.10
|
0.38
|
0.02
|
|
0.723
|
400
|
50
|
| |
|
|
|
|
|
|
|
|
|
|
|
Actual Analysis weight %
CEV(IIW) = C + Mn/6
+ (Cr+Mo+V)/5 + (Ni+Cu)/15
Producing
a quality weld in wear resistant steel
The processes used for welding wear
resistant steel typically include; SMAW, FCAW, MCAW, and SAW. Regardless of the process used, the primary
objective of the resulting procedure is to ensure that the high hardness of the
steel and joint be maintained, while avoiding cracking and weld defects.
1 - Maintaining Hardness:
To maintain the hardness (and
associated wear resistance) the aim of a weld procedure is to create a weld
where the heat affected zone of the weld is as hard as the base metal but not significantly
harder.
To achieve this result, weld
procedures are written to limit the heat input during welding, with limits selected
based on the thickness of the material. To ensure appropriate hardness levels
in the weld deposit an alloyed filler metal is used. Welding with mild steel
consumables will produce lower hardness levels in the joint. If this approach
is used, the hardness in the joint is then controlled by the amount of dilution
between weld metal and base metal.
2 – Avoiding Cracking:
A common defect when welding wear
resistant steels is cracking of the base metal next to the joint. This is caused by a mechanism called Hydrogen
Induced Cold Cracking (HICC). There are
3 primary components of a weld procedure (all are critical) used to avoid HICC:
Preheat - To avoid the potential for hydrogen
induced cold cracking, manufacturers typically will provide a recommended
preheat temperature that is based on the hardness of the steel and thickness of
the weld joint. This preheat is used to lower the cooling rate and help remove
hydrogen.
Consumable Selection - Low hydrogen consumables should be specified
by the weld procedure, which means proper storage is essential. Diffusible
hydrogen levels can increase significantly in consumables stored in an open
package out of an electrode oven, even over a period of a couple hours.
Weld pass Selection - For multi-pass weld joints it is recommended that the last weld pass does not touch the base metal. A “temper bead” welding technique may be used as well to avoid HICC and still maintain high hardness values in the heat affected zone.
The Importance of Following
Procedures
Unlike welding mild steels - a
relatively forgiving process - wear
resistant steel is highly sensitive to variations in the weld
procedure. It is critical that welders,
supervisors, and others involved in the production process be aware of and follows
the approved procedures at all times. The investment in training will pay for itself in reduced rework, and
longer service life.
