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Welding Tool Steel with Special Precautions
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Welding-Tool-Steel presents problems because of limited Weldability.
It is therefore a challenge to find suitable solutions if welding is needed.
Expensive tools may become unusable for any one of many possible causes.
It may then be necessary to weld them for saving the investment.
It is best to prepare a plan of action for deciding what is needed, how to select the process and the heat treatments.
First: What are Tool steels?...
Tool steel are iron base alloys of special composition, grouped usually according to their most important function like:
- Cold work tool steel, used for punches, shearing blades, forming tools, may be made of any one of the tool steels identified by the quenching medium used to develop hardness, listed here in the order of increasing hardenability:
- Water hardening tool steels, (Wx, where x is a serial number)
- Oil hardening tool steels, (Ox, as above)
- Air hardening tool steels (Ax and Dx as above).
- Shock resisting tool steels, used for hammers, chisels (Sx, where x is a serial number)
- Hot work tool steels, used for dies for casting and forging (Hx)
- High speed steels, used for machining tools (Tx for Tungsten based and Mx for Molybdenum based, x is a serial number).
- Other special purpose tool steels
Welding-Tool-Steel is generally required for the following purposes:
- Assembly of a composite tool made of simpler elements
- Modification of a tool to suit different requirements or to repair mismachining
- Rebuilding of a worn out surface or edge
- Repair of a broken, cracked or damaged tool
- Improvement of properties on a specific location of a tool built of a simple steel.
Machining of Tool Steel is preferably performed in the annealed, softest condition.
Also Welding-Tool-Steel should be done in soft condition, except that this is not always possible.
Tool steels are later processed by heat treatment to develop their most useful characteristics for the purpose intended.
One of the most important characteristics to remember when selecting a tool steel for a specific application is its hardenability.
This means the capability of displaying the required properties (hardness, toughness, wear resistance etc.) upon suitable heat treatment.
On this feature see our Hardenability page.
Different materials reach their optimum qualities at different quenching speeds from the hardening temperature.
Cooling a mass of hot material can be performed only by cooling its external surface.
The material thermal properties will determine the maximum cooling rate attainable.
That means that the size of the tool imposes limits to the attainable speed of cooling from austenitizing temperature and to the corresponding microstructures obtained.
Therefore a material perfectly adequate to develop suitable properties in a small tool may be unsuitable for building a massive tool.
That is because it may be impossible, with that material, to obtain the required hardness with a regular heat treatment at the maximum cooling rate.
Tool steels include a variety of different compositions with most varied types of weldability, that is the ease or the difficulty with which they can be welded.
However tool steels are mostly highly alloyed and with considerable amounts of Carbon.
Therefore, in order to reduce internal stresses and to eliminate crack formation, they need to be acted upon by welders with special welding skills.
Furthermore suitable procedures for Welding-Tool-Steel may require application of definite preheating and postheating.
It is generally preferable to weld tool steels in annealed condition, although sometimes it may be impractical to perform an annealing treatment as a preparation for welding.
Shielded Metal Arc Welding-Tool-Steel is the preferred process because of its advantages.
Among them the large selection of available filler materials, and the fact that the process is most versatile.
The solid slag shielding provides adequate protection and may retard cooling, a favorable feature.
Other gas shielded processes are viewed as less reliable if drafts are in the area.
Among the disadvantages we list
- the frequent interruption for changing electrode, (even more if using a thinner one),
- the need of slag chipping and cleaning,
- the difficulty of maintaining preheat temperature,
- the peening requirement and
- the variability of results.
For shielded metal (SMAW) arc Welding-Tool-steel one should follow the manufacturers' recommendations and those listed hereafter.
- The smallest size electrode suitable for the job should be selected.
- The surface should be prepared by machining or by grinding, with rounded sides grooves presenting a joint angle of at least 30o and such that the width at the bottom be greater than the electrode diameter.
- If a crack has to be repaired, stop holes should be drilled at both ends and all the cracked portion should be machined away, with smooth transitions.
- For edge building or rebuilding it may be advantageous to use a support copper or graphite bar to contain the molten filler metal.
- Absolute cleanliness is most important for the work and electrodes, as it is the dryness of electrode cover.
- The lowest heat input possible should be used, and should be decreased for successive passes.
- If possible the work should be positioned at a slight uphill inclination so that the weld penetration be improved.
- Preheating should always be performed, possibly in a furnace or in a specially built insulated preheat box, at a suitable temperature before attempting to weld.
- Narrow beads should be deposited and then slag is to be removed by chipping and brushing between passes.
- Peening lightly is recommended on the hot weld to relieve stresses.
For selecting the most appropriate filler Welding-Tool-Steel, the following should be considered:
- the base metal composition, (from the tool steel designation),
- the condition of the tool (if annealed or hardened), and
- the service requirements of the welded area.
The importance of selecting a filler composition similar to that of the base metal, for Welding-Tool-Steel is greater for annealed tool steels.
These are generally to be welded during the tool manufacturing process.
The weld should respond equally well to the heat treatment providing hardness comparable to that developed by the base metal.
For Welding-Tool-Steel in hardened condition the selection is more difficult, however the range of possible materials is larger.
It is not recommended to repeat the hardening and tempering cycle, because of the risk of cracking.
If the volume required for Welding-Tool-steel is limited, one prefers to use a filler material (whose composition may be quite different from the base metal) which is self hardening upon cooling.
If the weld is needed in a non working area, then there is a wider selection either of low alloy steels or of stainless steels.
Specific filler selection.
All this said and explained we are still left with the all important task of selecting a suitable filler material for Welding-Tool-Steel.
A simple and comprehensive Table showing us which is which and when to pick what is simply not available.
The problem is that most tool steels are specific developments of individual manufacturers.
These may not be ready to disclose all they know about their products.
We may still want to know or inquire which general class the steel proposed belongs to.
Also the large variety of applications and of requirements means that different Welding-Tool-Steel solutions may be required in different situations, even on the same tool steel.
The same problem we face with electrode manufacturers, where the issues are possibly even more complicated by the limits imposed by the cover materials of electrodes.
An Article on the Selection of Filler Metals for welding Tool Steels is published in the August 2006 issue of Practical Welding Letter No. 36.
To read the article click on PWL#036.
An Article on Preheating Alloy and Tool Steels was published in Section 2 of Issue 37 of Practical Welding Letter for September 2006. To read the article click on PWL#037.
An Article on Cryogenic Processing was published (7) in Issue 57 of Practical Welding Letter for May 2008. To read the article click on PWL#057.
A new page was introduced (8) in Issue 94 of Practical Welding Letter for June 2011, at PWL#094. To see the new page click on Flood Welding.
To read all the Articles as they are published please Subscribe. You will also receive a bonus book on Practical Hardness Testing Made Simple.
It may be helpful.
Tip!: The best course of action would then be to try to recruit the services of the professional personnel (metallurgical experts) of the two sources (of steel and of filler metal).
Let them come up with their proposal for specific Welding-Tool-Steel for the intended application.
We should know the name or at least the class of the tool steel and where it was purchased.
If we do not know, then we should obtain its chemical composition by proceeding with Material Identification.
Or by submitting the tool or a small chip thereof (inquire on the size needed) to a suitably equipped Laboratory for identification.
We should also determine the Vickers or Rockwell hardness of the tool we are going to weld unless we know for sure that the material is still in the annealed state (as purchased).
Let us remind you that you are entitled to a FREE download of our book on PRACTICAL HARDNESS TESTING MADE SIMPLE, as explained further down in this page.
Then the filler metal supplier of our choice should be asked which of their filler materials would be recommended for Welding-Tool-Steel of the kind selected for the intended application.
We should obtain also the maximum of information available.
If possible we should make a weld test on a small specimen reproducing the original material and condition, and the proposed filler electrode.
The result will tell if the choice was adequate or not and if a more elaborate welding procedure should be developed.
Preheating temperature to prepare for Welding-Tool-Steel should be selected with care, with reference to the original tool steel and to the condition, if annealed or hardened.
See our page on Weld Preheating.
As a matter of orientation it could be similar to the suggested tempering temperature for the steel.
Thin materials require lower temperatures than thick ones.
If Welding-Tool-Steel is performed in several passes, inter-pass temperature should be maintained and restored as necessary between passes.
Any visual defects like arc strike marks or end crater, appearing on the surface upon completing the weld must be repaired immediately, before the tool cools down and before post heating is applied.
For Welding-Tool-Steel on annealed materials, one should soften the weld by annealing at the recommended temperature for the steel (and then furnace cooling).
Alternatively a sub-critical annealing treatment should be performed.
This consists in keeping the welded tool for quite a long time at a temperature just below that of austenite transformation and cool in air before performing machining and finishing.
Full heat treatment as required for the tool steel involved will follow.
Post weld heat treating is performed after Welding-Tool-Steel to relieve stresses.
Usually the heat is applied again after letting the work cool uniformly somewhat, but before it reaches room temperature.
Postheat temperature should not exceed the tempering temperature for the steel.
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