Toolweld, Inc Blog

Recently I was asked in an email whether NAK 55 can be welded. Below is my reply.

“As you know, Nak55 is a Japanese steel used as a semi replacement (or their version) for P20, although it’s harder, more polish-able, and it can be machined faster. We have a few customers that work on Japanese molds that use Nak55. To answer your question: Yes, it can be laser welded (I have done it), but there some pitfalls you have to be aware of.

NAK 55 has sulfur in it which can interfere which the welding process. What actually happens is that after a long repetition of welding, you will see the sulfur crystallize on top of the weld bead.In addition, the edges of the weld can crumble which it not good when it is going to be polished. I have learned through experience to laser weld in short passes–with narrow beads–and use what I would can a “sink pass” to cover any crumbling. The suppliers for NAK 55 will, of course, insist you use Nak55 filler; however, if its a repair (light welding of shut off areas and such) I use 410ss, which gives me smoother touch. If it’s a heavy buildup, new tool, or a polished surface then I would consider Nak55 filler. I have used 410ss even on polished areas and it’s worked out fine.

TIG welding requires different parameters.My main advice to is the longer you TIG weld Nak 55 the more problems you will have with the sulfur in the steel. You can actually see a white sulfur pit show up at the end of weld passes. This will infer with the weld so you need to take a punch and pop the sulfur off. A way to stop sulfur pits is to weld backwards in small beads, and the manufacturer will most likely suggest this procedure.

Here is the important thing:  When you welding Nak 55 you need to immediately follow the post heat procedure (see the link) if the mold has a polished surface. The post heating procedure is imperative to the success of NAK 55, although I think a lot of shops bypass this for the sake of time.”

http://www.lindquiststeels.com/documentation/nak55.pdf

The short answer is no.

Of course, this requires an explanation. Our definition of micro-TIG welding is using a TIG welder under a microscope via low amperage (1-50 amps) and a small diameter filler (.005″ to .020″). Since copper and aluminum need an incredible amount of amperage to melt, it’s impossible to pre-heat a piece high enough to melt at low amperage. Even if you did pre-heat the piece high enough to achieve the desired amperage range, the piece would lose structural integrity and it would deform prior to welding–there is a fine line between achieving a proper melting temperature and having it be too much. Yes, you could technically use micro-TIG (meaning under a micr0scope with TIG), but you would to use a much higher amperage and it would require at least at .035″ filler. This would defeat the purpose of welding small.

There are some other problems: Copper based alloys and aluminium are suggested to welded under AC (alternating current)–a cleaning action which is critical for aluminium. (There is a tool welder’s trick for welding aluminum under DC–direct current–that I will discuss in a later post.) There are two ways to weld these two alloys: regular TIG and laser welding.

Regular TIG (400 and above amps) will, of course, increase the size of the weld deposit because of the power required to melt. If you absolutely need a micro-weld size, then you have the option of laser welding. Laser welding is a pulsed, concentrating its melting power to a miniature area. Bare mind that laser welding is also timely and it has problems regarding ease of access.

A few years ago there was an article done about us in a trade magazine called Practical Welding Today.

http://www.thefabricator.com/article/arcwelding/9-1-1-weld-emergency

 

Since there is little to no decent information regarding the tool welding trade, I decided to contact this magazine in the hopes they would do an article about us. Thankfully they did; and here it is.

Some key points as to what filler materials a mold welder uses.

• Suppliers do no offer individual filler rods that match every die and mold steel on the market. When the exact match is not available, the welder will match the same hardness and general composition as close as possible.

 

• When welding on a new “soft” tool,” every effort should be made to match the composition of the base material. This way the welded area will achieve the same hardness as the base material after heat treating.

 

• On larger repairs, a base layer of softer rod can be used to give ductility in the weld deposit, which reduces the possibility of future stress cracks in the welded area. This softer base can to topped a  moderate, or matching hard rod, depending on the customer preference and  how the tool is used.

 

 

 

Here are some basic procedures for preparing a piece for welding. Following this will result in the best possible outcome.

• Remove all oil and grease.

When investigating cracks, you should keep two points in mind:

1) Cracks in the weld area of a pre-heated tool is not common occurrence. Dies, however, are the exception.

2) Good mold steel rarely cracks and bad mold steel cracks despite all precautions taken.

If a crack is exposed after the welding process, there are many reasons as to why this occurred and below are some of the most common.

1) There were existing cracks already which were too small too see, but the heat generated from welding open them up.

2) Something went wrong in heat treating, causing super hardened areas.

3) You were welding in a die steel. The reasons for this is because of its high hardness and brittleness. It is imperative the dies be pre-heated and post-heated to help lower the amount of cooling contraction during the welding process.

4) When a heavy weld build up is performed (.o60″ or higher), the stress caused by numerous weld passes can result in cracking. Post-heating and peening during the welding process are the best defenses against cracking.

5) Welding into a hard, brittle EDM re-cast layer (especially in corners) can be a problem. Sometimes EDM re-cast layers can have the same hardness as carbide which can result in a cracking nightmare. Lightly grinding down the re-cast layer can minimize this problem.

One of the most common tasks for a welder is to weld an existing crack on a tool or die. When you have a crack in a mold, the first question you should ask yourself is: Why did this piece crack in the first place? By asking yourself the previous question, one can further ask: Will welding the crack solve the problem completely? Fixing a crack by welding is often considered a temporary solution by welders.

Here’s the best way to fix a crack by welding:

• Clean all debris out of cracks. The black soot that develops during welding will keep the welder from seeing the weld detail and the melting debris will result in porosity of the weld; this will result in more cracks later.

Sinking is one of the biggest problem a welder faces when welding on molds. A welder’s definition of sinking is when the steel cools and shrinks below the original level on the heat affected zone of the weld. Anytime a deep hole or area is filled, the worse sink will be. Basically, going into a deep slot requires a lot of amperage for a proper melt; as a result, there is a greater contraction of the welding zone–causing greater sinks. In most forms of welding, this means nothing; but in plastic injection molds the result can be tragic, since it will change areas of the part cosmetically. What happens is the contraction of the weld shrinks and leaves a deep zone around the heat affected zone in a ring like shape, which all show up on a plastic part.

In addition to sinks, they are undercuts. An undercut normally occurs when weld on a sharp edge does not clean up to the existing form of the material after machining, resulting in a notch in the steel which can be felt by running your fingernail across the weld area to the original steel. Welders have techniques which stop undercuts which I will explain in a later post.

There is some ways tool welders try and stop sink:

1) Proper pre-heating (see blog on that subject) slows down the rapid cooling of the weld zone, thereby reducing sink.

2) Using only enough amperage to achieve a melt while keeping the overall heat as low as possible. Again, low amperage is achieved by properly pre-heating! When the mold is properly pre-heated, the welder can use lower amperages because he/she no longer has to compensate for the coolness of the piece.

3) Peening (using a hammer and punch) the hot weld will push down the weld and rise up the perimeter material–reducing sink. Again, pre-heating is an important part because it slows the rapid cooling and the welder is able to peen quickly and efficiently.

4) Laser welding (as opposed to tig welding) the heat affected zone will eliminate sink. Under certain conditions this procedure can be applied and it’s a life saver.

Pre-heating is one of the most important procedures you need to do prior to welding. However, the real reasons for pre-heating are different from what people generally think. In the molding industry, it’s generally assumed that tool steels should be pre-heated before welding to minimize cracking. I’m going to get geeky here for a moment: While the above is accurate, the REAL reason why mold welders pre-heat is to prevent the martensitic transformation (excess hardening in the areas around the weld area. This hardened area can result in a future problem (heat check cracking), as the mold rapidly heats and cools while in service. In layman’s terms, not pre-heating causes embitterment in areas next to the weld. In addition to all of this, pre-heating also slows down the weld contraction (the rapid heating and cooling of the weld puddle produced by welding) which allows the welder to peen the weld and minimize the sink caused in the heat affected zone. In the tooling industry, sink can become a large problem since too much sink can because problems and show up cosmetically in a molded part. Technically, pre-heating should be at the temperature above the martensitic range, but below tempering range (where the overall steel hardness can be affected). These two ranges can be very close so it is always best to err on the side of caution and keep the temperature 150 degrees lower than the martensitic to keep the mold hardness from drawing down. Mold and die steels all have different martensitic and tempering parameters, so this where the experience of the tool welder comes in to play.

Basic pre-heat guidelines

420SS–400 to 500 F

H13–600F

P20–550 F

S-7–450-550 F

**This being said, now it’s time for a reality check.**

Tip One: While tool steels should be pre-heated, the possibility of cracking around or near the welded area is actually low. However, this does NOT apply die (hard) steels, which needs to be properly pre and post heated because it can crack.

Tip Two: We live in the real world, so when a mold fails in service, time becomes paramount and the idea proper pre-heating gets thrown out the window when your customer needs something fast, as they always do. Chances are that if a customer waited for his job, then it wasn’t properly pre-heated. It’s a dirty little secret between welders and mold makers, because to do it properly takes time–and you rarely have the time. In this situation, the experience of the welder comes into play, knowing exactly how to adjust the pre-heating temperatures to complete the job in a timely manner and achieve a high quality. But, anything welded should be stress relieved (post heated) by an experienced heat treatment specialist. The procedures to “push” a job along by no means replaces properly timed pre and post heating.