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In This Issue

1   About Preheat and Postheat Procedures
2   Inventor of the Fahrenheit Scale
3  Postalloy® 285SPL and WC
4   Humor: Drivers License
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About Preheat and Postheat Procedures

Preheating and Postheating procedures can be confusing, but some very basic rules should help.  We will confine our discussion to iron base martensitic deposits and base metals.  We will first look at Preheating and Interpass Temperatures in this issue and address Postheating procedures in the next issue.

Preheating and Interpass Temperatures:  I have combined preheating and interpass temperatures together because they basically go hand in hand.  Rarely does and interpass temperature differ from the preheat temperature. Why do we preheat?  Here are a few of the reasons.

1. To ensure uniform hardness throughout the deposit.
2. To help prevent excessively fast cooling rates.
3. To help drive off moisture from base materials prior to welding.
4. To help prevent excessive distortion.
5. To aid in bead tie-in and improve bead appearance.

As weld metal cools from the liquid state it passes through two very basic transformation structures (Austenite and Martensite), providing the cooling rate is fast enough. Austenite is soft and tough and Martensite is hard and brittle. Ultimately we want the Martensitic structure to help resist wear. It would therefore seem natural to quench each bead to ensure we form Martensite, and that would certainly happen if we didn't apply any preheat. Each bead would be quenched. However, quenching each bead has some very severe consequences. First, as each subsequent bead is applied the heat from that bead would temper and soften the previous bead, leaving a very non-uniform hardened deposit. Only the last bead deposited would have maximum hardness. Secondly, quenching each bead sets up the structure for Hydrogen Cracking, which we covered in a previous issue.

It is obvious that quenching each bead is out of the question. So, we must decide on an alternative. Martensitic steels are most successfully deposited at a temperature high enough to avoid any hardening. That temperature is known as the Martensite Start Temperature, or MS. The correct procedure is to deposit all beads above this temperature, and then bring the whole deposit down at one time in a proper postheat procedure. If all goes well, we will have a uniformly hardened deposit.

The MS temperature is quite dependent upon carbon and other additional alloys. Generally speaking, the higher the Carbon content, the lower the MS temperature. The exact MS temperatures for many steels can be found in metallurgical books. Postle can also supply these. Most steels can be safely welded at temperatures above 500ºF, but a quick rule of thumb is as follows:

Preheat Temperature (ºF) = (Carbon Content x 1000) + 100

For 4340: 

Preheat Temperature (= (0.40 x 1000) + 100 = 500ºF

Please keep in mind that this is rough rule of thumb and that exact temperatures may vary up or down from these figures.  

Now that we have our formulas, it is important to take into account both the base material and the deposit chemistry.  If you apply the formulas to both, it is important to use the highest temperature 

For example Postalloy® 2898 on 4340:

4340 = 500ºF

2898 = 600ºF

In this case 00ºF is the ideal Preheat Temperature.  

It should also be kept in mind that during welding the interpass temperature will most likely rise.  Because of this, the actual preheat temperature could be lowered to perhaps 550ºF.  The exact temperature to which it can be lowered will depend upon the size of the component to be hardfaced. Thinner components such as agricultural tools can be as low as 400ºF.

Applying preheat will also help drive off moisture and oil, which are both bad actors when it comes to hydrogen embrittlement. 250ºF is probably a good number to apply, providing formulas do not dictate higher temperatures. 

Applying preheat will also aid in distortion prevention.  As the base material is heated, it expands in all directions, bringing it closer dimensionally to the molten metal which is about to be deposited.  Granted it cannot approach the molten metal, but every little bit helps.  

Hardfacing materials usually have better bead tie-in and appearance when applied to hot base materials.  Deposit fluidity is much better, allowing for easy manipulation of the puddle.

Preheating and Interpass Temperatures is just part of the story.  Cooling rates and Postheating procedures are just as important, and we will address this in the next issue. 
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"Inventor of the Fahrenheit Scale"

Gabriel Daniel Fahrenheit

Gabriel Daniel Fahrenheit was born in 1686 in Danzig, Poland, and was raised in Holland and England. After failing as a merchant, he developed an interest in meteorology. Among his contributions were a pumping device for draining the Dutch polders; a hygrometer, for measuring atmospheric humidity; and a thermometer that used mercury instead of alcohol.

Of course, a thermometer really does no good unless there are numbers on it, so he developed a temperature scale, too. On his scale, 32 degrees is the freezing point of water, and 212 degrees is the boiling point. This scale is called Fahrenheit. Later a Swede from Stockholm would suggest a more logical scale, with a temperature range of 0-100. His name was Anders Celsius.

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Postalloy® 285 and Tungsten Carbide (WC) 

A very old practice of dropping chips of Tungsten Carbide (WC) into a molten puddle of a mild steel electrode or wire is still in popular use today. This often produces a high carbon matrix that sometimes becomes so brittle it endangers the WC chip itself. There is evidence that the WC actually falls out during the wear process. A very successful alternative to the mild steel is to use Postalloy® 285SPL as the matrix wire. This is a highly alloyed manganese deposit that is very carbon tolerant and very tough as well. It does not embrittle as the mild steel deposits do and therefore does not put the WC chip in jeopardy. It work hardens and there is some evidence that it actually wets the WC chip better than a straight carbon steel. All of this translates to LONGER COMPONENT LIFE. Some have reported as much as double the life. Now that's impressive. If your operation is using a mild steel wire or electrode, ask us about substituting Postalloy® 285SPL. I don't think you will regret it.

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When I went to get my driver's license renewed, our local motor-vehicle bureau was packed. The line inched along for almost an hour until the man ahead of me finally got his license.

He inspected his photo for a moment and commented to the clerk, I was standing in line so long, I ended up looking pretty grouchy in this picture.

The clerk looked at his picture closely, and reassured him, "It's okay. That's how you're going to look when the cops pull you over anyway".

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