metallurgy for welders ,cooling rate, heat input, pre heat, inter pass temperature

7/27/2019 Metallurgy for Welders ,Cooling rate, heat input, pre heat, inter pass temperature

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Metallurgy for welders

Addressing 4 key issues

By Keith Packard

May 25, 2012

The goal when welding any material is to change its microstructure as little as possible and to

preserve its mechanical and chemical properties. To achieve this you must be able to

determine its weldability, control the heat input, and prevent rapid cooling.

Metallurgy affects the way you approach applications every day, as well as the equipment

and filler metal you use. Once you recognize a materials weldability and the way it reacts to

heating and cooling, youll have a greater chance of successfully completing the job.

Websters dictionary defines metallurgy broadly as the science and technology of metals.

But in practical terms, metallurgy affects the way you approach welding applications, the

equipment and filler metal you use, and the challenges you face throughout the welding

process. Not surprisingly, the metallurgical properties of a piece of metalits mechanical

strengths and chemistryalso determine how well, or if, it can be welded. From carbon and

sulfur levels to tensile strength and the manner in which a given material is processed or

reacts to heating and cooling, each element factors into the success or failure of a welding

application.

When welding any material, your goal is to resist changing its microstructure and to preserve

its mechanical and chemical properties. To do this, every welder should consider these key

metallurgical issues before starting a job.

1. Is It Weldable?

Its critical that you know as much as possible about the material before striking an arc. Ask

yourself, Is it weldable? Weldability refers to the ability of two pieces of material to bewelded together and still maintain the desired mechanical and chemical properties for the

application. A few conditions can affect a materials weldability, and there are a few things

you must be able to do to ensure a successful operation.

Identify the Material. Figuring out the weldability of materials can be difficult at times. You

may be required to weld a part without knowing what material it is. Or you might receive a

part from a customer who has not specified that information. Chemistry tests and spark tests

are recommended to identify the metallurgy of the material before proceeding with the

welding process.

Understand Special Welding Requirements. Not all materials lend themselves to being

welded, and some require special precautions before, during, or after the process. For
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example, materials such as resulfurized steels have high levels of carbon, sulfur, and

phosphorus, making them notoriously difficult to weld because they are highly susceptible to

cracking. Many types of chrome-moly steel (4000, 4100, and 4300 series) also have higher

carbon and chrome levels than carbon steels and are similarly prone to cracking if you dont

follow proper welding procedures. These include selecting the appropriate filler metal and

employing preheat and postweld heat treatment (PWHT) when necessary.

Evaluate Joint Design and Preparation. In some cases, the joint design may affect your

ability to access the joint, which in turn affects the materials weldability by limiting the

opportunity to perform a clean weld with proper penetration. Weldability can be affected

further by joint preparation. For example, materials that are carbon arc gouged can

accumulate residual carbon on the surface that can lead to cracking. Or a piece of material

that has been machined may have residual machining fluid that could generate porosity.

Paint, oil, and grease can also affect a materials weldability, so the material should always be

properly cleaned prior to welding.

Given these possible conditions, you must make the proper accommodations to ensure thatthe weld can still be made successfully. Cleaning materials properly and implementing proper

weld procedures can help address issues with weldability. Similarly, selecting filler metals

that are not crack-susceptible and have good ductility or toughness properties (depending on

the needs of the given material) is also important.

2. Controlling Heat

High heat input during welding can affect the mechanical properties of a material adversely.

When a weld joint becomes too hot, it dissipates the heat quickly, causing internal stress in

both the weld and the base material. Similar stresses can cause the two pieces of material topull apart after cooling. Both situations, singularly or in combination, can lead to cracking.

Cracking of this nature is quite common in chrome-moly steels with high chrome and carbon

levels, such as 4000 series materials; however, it can occur in most any material type.

High heat input can also lead to distortion, which typically occurs in thin materials and those

that are highly restrained because of a particular weldment design. The localized heat input in

the weld joint causes the material to change shape when it cools. You can resolve this

problem by clamping the part and prebending it in the opposite direction, or by controlling

the bead sequence. A series of small stringer beads also can help minimize distortion because

it reduces the amount of heat going into the weldment. In some materials like quenched andtempered steels, high heat input can cause the material to soften and weaken.

For all of these reasons, it is critical for you to monitor how much heat you are putting into

the weld joint during the welding process and control it accordingly. The equation you can

use to determine heat input is:

Amps Volts 60/ Travel Speed (in inches per minute) = Kilojoules per Inch.

You can also use a temper bead effect to help refine the microstructure of the grains in the

weld and provide it with good strength. To create a temper bead, add two to three extra weld


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beads on top of a weldment, which causes extra heat to go into the weld bead below, thereby

tempering it.

Preheating is another option. Bringing the material up to a specified temperature before

welding can help reduce the residual stresses in the material and prevent it from cooling too

quickly, which causes changes to the materials microstructure that lead to cracking,distortion, and softening. Monitoring and controlling interpass temperatures on multipass

welds is also critical, as is implementing slow cooling procedures.

Preheating is key in controlling heat input and preventing rapid cooling. Always follow the

proper PWHT recommendation for your application.

3. Preventing Rapid Cooling

In conjunction with controlling heat input, you also need to control the rate at which the weld

cools. Uncontrolled cooling can lead to many problems both within the weld and in the base

material. In particular, rapid cooling changes the materials microstructure. Instead of small,

fine grains that are evenly dispersed throughout the weld, the grains become larger,

decreasing crack resistance.

Rapid cooling most often occurs in tandem with high-heat-input conditions, as discussed

previously, but it can also happen without them. For example, if you weld a thick piece of

material without preheating, it becomes a large heat sink. Even though it may not heat up

very much, it sucks the heat out when the weld is complete, which causes rapid cooling

within the material.

As with high heat input, rapid cooling may lead to distortion, increased hardness, and

decreased ductility. In some cases, it can also induce hydrogen cracking, which is often

referred to as cold cracking or heat-affected-zone (HAZ) cracking. This type of defect begins

in the base metal and passes transversely into the weld as it progresses. It is the result of both

residual stresses and the presence of diffusible hydrogen in the weld, and changes in the

microstructure of the material.

To prevent rapid cooling, preheat the base metal and control interpass temperatures on

multipass welding applications. Preheating offers the additional benefit of allowing the arc to

penetrate the weld joint more readily. You can also perform PWHT, holding the finished

weld at a prescribed temperature for a period of time via a process like induction or furnace

heating. PWHT helps relieve residual stresses and it drives diffusible hydrogen from the

weldment to help minimize the chances of cracking.

4. Matching Filler Metals


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Selecting the appropriate filler metal can play a role in overcoming challenges associated

with a materials chemical and mechanical properties. As a rule, most applications require

matching filler metal tensile or yield strength to that of the base material. The word

matching here is because the two strengths may not be exact.

In some cases, it may be desirable to undermatch the strength of the filler metal to the basematerial. Undermatching can be beneficial because it helps increase toughness and ductility

and may help minimize the residual stresses in the weldment.

While there are additional metallurgical considerations that you should become aware of,

determining weldability, controlling heat, preventing rapid cooling, and matching filler

metals are the main ones. To prevent changing the microstructure of a weld and the materials

it holds together, always be mindful of prescribed procedures, and know what type of

material you are welding before beginning any process.

metallurgy for welders ,cooling rate, heat input, pre heat, inter pass temperature

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