Note: Descriptions are shown in the official language in which they were submitted.
~065652
This invention relates to ~eld metal deposits suitable for
forming hard facings on metallic articles such as $or example ~ork rolls
or backup rolls for use in a rolling mill for metal.
It is well-known practice to use arc welding to provide various
articles, including rolls for hot rolling mills with a facing of a wear-
resistant alloy. Below are listed some examples of weld metal compositions
which have been used in facings on rolls for hot rolling mills:-
Example No. 1 2 3 4
Percent Percent PerceP.t Percent
C 0.22 1.10 0.05 0.08
Si 1.0 1O25 0.30 0.55
Mn 0.5 1.10 1.50 1.30
Cr 5.0 7.70 4.80 2.5
Mo 1.1 1.00 0.63 1.0
V 0.3
W - 1.60
Fe remainder remainder remainderremainder
The weld deposit of Example 2 has the best wear resistance
but also has a strong tendency to form hot cracks ~solidification cracks)
and therefore can in practice be used in such cases only when hot cracks
in the weld deposit are allowable, for instance in rollers for roller
conveyors and other rollers subjected to comparatively low working
pressures. Weld deposits according to Examples 1, 3 and 4 and similar
alloys are less subject to cracks and have been used in practice as a
coating material for work rolls in hot rolling mills, that is, rolls
required to perform a hot metal rolling operation. Economical considerations,
however, frequently exclude the use of the known deposit
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alloys, the wear resistance or useful life obtainable with the resulting
facings proving insufficient to weigh up the cost of their use for the re-
pair or the manufacture of rolls for hot rolling mills.
- It is an object of the invention to provide an improved weld metal
deposit particularly suited for hard facing purposes which has little or no
tendency to cracking and has a satisfactory hardness combined with a high
resistance to wear. A more particular object is the provision of a weld metal
deposit of a composition particularly suited for the hard facing of hot
; working tools, that is, tools used for the working of metals at elevated
temperatures, for instance drop forging dies, pressing dies, extruding press
nozzles, dies for die casting machines, mandrels for piercing mill, mandrels
for the hot drawing of tubes and rolls for hot rolling mills.
In the accompanying drawings, Figure 1 is a diagram the shaded
area of which indicates a prescribed relationship between the contents of
niobium and carbon of the weld metal deposit according to the invention, and
Figures 2 to 4 are tempering diagrams showing the hardness as a function of
the tempering temperature for three different weld metal compositions accord-
ing to the invention.
The present invention provides a weld metal deposit applied as a
coating on a hot working tool having substantially the following composition:
Percent by weight
C 0.55 to 2.0
Si 0.2 to 4.0
Mn 0.2 to 3.0
Cr 4.0 to 6.5
Nb 2.5 to 15.0
Strong carbide formers other
than niobium selected from
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the group consisting of tung-
sten, molybdenum, vanadium
and titanium 0 to 3.5
Ni 0 to 0.5
the remainder being substantially iron with or without incidental impurities,
the contents of carbon and niobium being in a ratio of one to another as
defined by the area indicated in Figure 1 of the accompanying drawings.
In particular, the present invention provides a weld metal
deposit applied as a coating on a hot working tool having substantially the
following composition:-
Percent by weight
C 0.65 to 0.70
Si 0.5 to 1.0
Mn 0.75 to 1.25
Cr 4.0 to 6.0
Nb 3.2 to 4.0
Mo 1.0 to 1.25
remainder substantially iron with or without incidental impurities.
The present invention also provides a weld metal deposit appliedas a coating on a hot working tool having substantially the following compo-
sition:-
Percent by weight
C 1.0 - 1.2
Si 2.0 - 2.5
Mn 2.0 - 2.5
Cr - 4.0 - 6.0
Nb 8.0 - 9.0
Mo 0.9 - 1.1
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remainder substantially iron with or without usual impurities.
The present invention further provides a weld metal deposit
applied as a coating on a hot working tool having substantially the following
composition:-
Percent by weight
C 1.5 - 1.7
Si 2.75 - 3.25
Mn 2.0 - 2.5
Cr
Nb 11.5 - 12.5
Mo 0.7 - 1.0 ;
remainder iron with or without incidental impurities.
~ As the niobium metal or ferroniobium commercially available
usually contains some tantalum as an impurity, the weld metal deposit of
the invention usually will containg some tantalum.
In apreferred form of the invention, the weld metal deposit con-
tains 0.6 to 1.5 % of molybdenum. -
The weld deposit according to the invention may be produced by~
any suitable welding process, including the electroslag welding process.
Preferably, however, the weld deposit is produced by electric arc welding
with a consumable electrode which may be either a bare electrode (in sub-
merged arc welding and gas-shielded welding) or a flux coated electrode ~in
manual welding). The bare electrodes to be used in submerged arc welding
and gas-shielded welding preferably consist of a mild stell sheath enclosing
a filling of alloying agents compounded so as to provide, in combination with
the mild steel sheath, the required composition of the weld metal deposited.
The filling may also contain specific deoxidising agents. The filling com-
pound of an electrode for gas-shielded arc welding also may include a pro-
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portion of fluxing agents to provide a flux shield supplementing the protec-
tion of the molten weld metal afforded by the shielding gas. A flux coated
electrode for manual welding may have a mild steel core, the required alloy .
constituents of the weld deposit being supplied as constituents of the flux
coating, which should preferably be of the lime basic type.
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~065652
When the weld metal deposit of the invention is produced by
the submerged arc welding process, preferably a sel$-releasing flux of the
neutral or basic type should be employed. The term ~'self-rele~sing"
indicates that the layer of solidified slag produced by the ~elding process
does not adhere to the weld metal deposit but peels off in large fragments.
The deposited ~eld metal according to the invention exhibits in
spite of its relatively high content of carbon a high degree of safety
against the occurrence of hot cracks in the welding operation. This
favourable property can be explained as an effect of the niobium content
of the alloy, which according to the invention has to be suited to the
carbon content. Investigations have shown that the niobium combines with
a substantial part of the carbon of the alloy to form niobium carbides.
These carbides start to form at an early stage of the solidification
process and consequently prevent the segregation of a liquid rich in
carbon which is a condition for the occurrence of hot cracks.
The weld metal deposit of the invention has a strength and
toughness adequate for the very testing conditions to which a facing of
a hot working tool is subjected. Moreover, the deposit has a superior
resistance to wear, including the wear exerted by the hot workpieces
upon the working surface of a hot working tool. The improved wear
resistance of the deposit is believed to be due principally to the
presence of a substantial proportion of niobium carbides in the metal.
The weld metal deposit of the invention is also very resistant to
thermal cracking ~sometimes referred to as "fire cracking" or "thermal
fatigue"). Thermal fatigue is caused by a gradual increase of the length
and ~epth of minute surface cracks in the working face of hot working tools
and can be represented by a graph showing the crack length as a function
of the number of thermal cycles to which the deposit has been subjected.
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With the weld deposit accord~ng to the invention, the rate of gro~th
of the thermal cracks is very~small, so that the useful lie of a facing
consisting of the ~eld metal depos-it o the invention is determined
mainly be the wear properties of the surface.
The chromium content of the weld metal deposit is important
for improving the hot strength of the steel (to which the resistance to
thermal cracking is principally due) and the oxidation resistance of
the steel. The chromium performs an additional important function in
imparting to the steel the metallurgical transformation characteristics
required for obtaining a desirable structure of the metal matrix of the
deposit.
In hard facing with steel alloys of the general type to which
the weld metal allo~ according to the invention belongs, that is, air
hardening alloy steels, it is well-known practice to preheat the workpiece
to be hardfaced to a temperature above the temperature Ms at which martensite
starts to form in the weld metal and within a temperature range in which
the austenite remains untransformed or is subjected to a minimum of
transformation during a period of sufficient length for carrying out the
entire welding operationO
A preheating temperature in the range between 400 and 500C is
preferred. These rules appl~ to the production of the weld metal deposit
according to the invention as well. Owing to the comparatively high
preheating temperature, the first layer of the weld will consist of a
; mixture of deposited metal with a comparatively high proportion of fusedparent material. As a rule, thereforeJ it will be necessary to deposit
at least two, prefercibly three layers on top of each other in order to
ensure that the top layer has a composition substantially identical with
the metal deposited by the welding electrode. It is possible, however,
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by the use of special techni~ues to make the dilution of the metal of the
first layer with parent material low enough to obtain the desired
composition of the facing By the deposition of a single layer.
The general range of compositions according to the invention
can be subdivided according to the carbon content of the alloy into three
groups having different metallurgical structures:
GroupCarbon content Structure of
Percent the matrix
1 0.55 to 0.8 martensitic/bainitic
2 0.8 to 1.2 predominantly martensitic
3 1.2 to 2.0 ferritic.
A specific example within each of the above three groups
will now be described.
Example I
Percent
C 0.65 to 0.7
Si 0O5 to 1.0
Mn 0.75 to 1.25
Cr 4.0 to 6.0
Nb 3.2 to 4.0
Mo 1.0 to 1.25
Fe remainder, with or without incidental
impurities including sulphur up to
0.04~ and phosphorus up to 0.04%.
A weld deposit of this composition has a Vickers hardness
which may vary between 300 and 60Q Hv, depending on the choice of the
tempering temperature. Fig. 2 shows a typical tempering diagram. For
some compositions within the range of this Example, the hardness may
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exhibit a maximum ("secondary" hardness) at a tempering temperature
of about 50a C.
Example II
Percent
C 1.0 to 1.2
Si 2.0 to 2.5
Mn 2.0 to 2 5
Cr 4.0 to 6.0
Nb 8.0 to 9.0
Mo 0.9 to 1.1
Fe remainder, with or without incidental
impurities.
A welding deposit of this composition has a hardness in the
range between 300 and 550 Hv. Fig. 3 shows a tempering diagram for a
welding deposit having a composition within the range of this Example.
Example III
Percent
C 1.5 to 1.7
Si 2.75 to 3.25
Mn 2.0 to 2.5
Cr 4.0 to 5O0
Nb 11.5 tb 12.5
Mo 0.7 to 1.0
Fe remainder, with or without incidental
impurities.
The hardness range for a weld deposit of this type covers
the approximate range of 340 to 380 Hvo Fig. 4 shows a typical tempering
diagram.
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All of t~e above Examples have been used successully for the
hard facing of large work rolls for hot rolling mills in the ollowing
way. The roll to be hardaced, which is preheated to a temperature in the
range between 400 and 5Q0C and maintained at this temperature throughout
the welding operation, is rotated while a welding head for submerged arc
welding is displaced slowly in the axial direction so as successively to
deposit a layer built up of adjacent turns of a spiral beadO At least two,
as a rule three, layers are deposited on top of each other. Welding
currents in the range between 300 and 1200 amperes are used. For
welding currents in the upper part of this range, the use of twin electrodes
~that is, a pair of single electrodes working in a common welding pool) is
preferred. It will be understood that the invention is not limited to
the above welding conditions, which are stated by way of example only.
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