INTRODUCTION
The
basic metallurgy terms which a welding supervisor or a welding engineer or a QC
dept. or a welding inspector should be familiar with is listed below.
- Heat Input
- Carbon Equivalent
- Heat Treatment
- Stresses
- Diffusion
- Solid Solubility
Generally the welding
can result changes in both temperature of the metal as well as the cooling rate
of the metal. During this time period the metallurgical changes occur on the
metal, these changes which happens in the metal must be understood thoroughly.
For example let’s
consider a metal to be welded, when the metal is welded heat is being generated
and is being transferred to the plate and the heat is dissipated up to a
particular level. As shown in the figure below let’s say the heat dissipated
has been divided into six different temperatures at 6 different region. The
temperatures are approximately 600, 500, 800, 1000, 1200, 1520 and the area
that was melted. During this process the factors which are considered is
- Maximum temperature up to which the metal is reached
- The time period spent at that particular temperature.
- The amount of time required to cool down.
During this process of
heating and cooling down there can be many different structures that can be
produced. Within the weld region the temperature reaches to its max and the
liquid form of metal is solidified through various phase regions. And in the
Heat Affected Zone (HAZ) the temperature does not reach to the melting point
but it attains some amount of temperature. The cooling rate in the HAZ region
is rapid. The welding condition plays a major role in formation of various
phases, these condition influence the cooling rate of the weld too. Some of the
welding conditions are use of pre heat, amount of heat input, base metal
thickness and carbon equivalent of base metal.
 |
Temperature Gradient on Metal During Welding Process |
HEAT INPUT
The
heat input can be defined as the ratio of welding current and welding voltage
by welding travelling speed. The can be expressed in the formula as shown
below,
Heat input = [Welding Current x
Welding Voltage x 60]
[Welding Travel
Speed (in/min)]
The
heat input is expressed in terms of joules/inch and travel speed in inches/minute.
And the 60 above the line converts minutes into seconds. Factors affecting heat
input and cooling rate,
Less
heat input & Higher Cooling rate
|
Smaller
Diameter electrodes, less current, Higher travel speed.
|
High
heat input & Lesser cooling rate
|
Larger
Diameter Electrode, High Current, Lesser Traveling Speed.
|
The
heat input is monitored to control the microstructural properties occurring in
the heat affected zone. The factor which effect the microstructure structure of
heat affected zone is pre heating. This pre heat reduces the cooling rate in
the weld as well as the HAZ which results in improving the ductility of the
material. When the pre heat is not used, the HAZ is relatively narrow and
exhibits its highest hardness. Depending on the alloy content martensite may be
formed. Whenever the pre heating is done with the material the HAZ zone becomes
wider which results in less cooling rate and it can undergo formation on
ferrite, pearlite and possibility bainite instead of martensite. Deciding the
pre heat temperature for a particular material is taken care by the welding supervisor,
engineer or QC dept. The cooling rate in the HAZ produce microstructures having
desirable properties.
 |
Heat Affected Zone (HAZ) |
CARBON EQUIVALENT (CE)
The
other most important factor to be considered is the amount of carbon content
present in the material. The carbon has more effect on hardenability. The
higher the carbon content the hardness of the steel is more. Other alloy
elements also provide hardenability.
CE = %C + %Mn + %Ni + %Cr + %Cu +
%Mo
6 15 5
13 4
Carbon
equivalent is an expression which helps to find the effect of various alloying
element on the hardenability of steel. This formula contains 0.5% carbon, 1.5%
Manganese, 3.5% Nickel, 1% Copper and 0.5% of Molybdenum. These CE value allows
to predict the approximate preheat temperature for the material. The
approximate range of preheat temperature according to the value of CE is shown
below,
Carbon
Equivalent
|
Preheat
Temperature
|
CE
< 0.45
|
Optional
|
0.45
– 0.60
|
200ºF
- 400ºF
|
CE
> 0.60
|
400ºF
– 700ºF
|
Basically
the size of the base metal also influences in cooling rate of the metal. The
thicker the size of the metal the faster the cooling rate occurs and for
thinner base metals the cooling rate is slower when compared to thicker ones.
These faster cooling rate affects the mechanical properties of the metal. In
order to slow down the cooling rate the thicker materials are being pre heated
for a long time.
HEAT TREATMENT
There
are some basic heat treatment processes which are applied to metals before
welding. Some of the heat treatment processes are mentioned below,
- Annealing
- Normalizing
- Quenching
- Tempering
- Aging
- Thermal stress relieving
This
heat treatment is done before the welding process which produces some specific
mechanical properties. Before welding one of the job is being heat treated and
kept under observation to notice down the parameters such as time and
temperatures.
Annealing:
- Annealing is a rather generalized term. Annealing consists of heating a metal to a specific temperature and then cooling at a rate that will produce a refined microstructure, either fully or partially separating the constituents. The rate of cooling is generally slow. Annealing is most often used to soften a metal for cold working, to improve machinability, or to enhance properties like electrical conductivity.
Normalizing:
- Normalizing is a technique used to provide uniformity in grain size and composition throughout an alloy. The term is often used for ferrous alloys that have been austenitized and then cooled in open air. Normalizing not only produces pearlite, but also martensite and sometimes bainite, which gives harder and stronger steel, but with less ductility for the same composition than full annealing.
Quenching:
- Quenching is a process of cooling a metal at a rapid rate. This is most often done to produce a martensite transformation. In ferrous alloys, this will often produce a harder metal, while non-ferrous alloys will usually become softer than normal. To harden by quenching, a metal (usually steel or cast iron) must be heated above the upper critical temperature and then quickly cooled. Depending on the alloy and other considerations (such as concern for maximum hardness vs. cracking and distortion), cooling may be done with forced air or other gases, (such as nitrogen). Liquids may be used, due to their better thermal conductivity, such as oil, water, a polymer dissolved in water, or a brine. Upon being rapidly cooled, a portion of austenite (dependent on alloy composition) will transform to martensite, a hard, brittle crystalline structure. The quenched hardness of a metal depends on its chemical composition and quenching method. Cooling speeds, from fastest to slowest, go from fresh water, brine, polymer (i.e. mixtures of water + glycol polymers), oil, and forced air.
Tempering:
- Untempered martensitic steel, while very hard, is too brittle to be useful for most applications. A method for alleviating this problem is called tempering. Most applications require that quenched parts be tempered. Tempering consists of heating steel below the lower critical temperature, (often from 400 to 1105 ˚F or 205 to 595 ˚C, depending on the desired results), to impart some toughness. Higher tempering temperatures (may be up to 1,300 ˚F or 700 ˚C, depending on the alloy and application) are sometimes used to impart further ductility, although some yield strength is lost.
Aging:
- Some metals are classified as precipitation hardening metals. When a precipitation hardening alloy is quenched, its alloying elements will be trapped in solution, resulting in a soft metal. Aging a "solutionized" metal will allow the alloying elements to diffuse through the microstructure and form intermetallic particles. These intermetallic particles will nucleate and fall out of solution and act as a reinforcing phase, thereby increasing the strength of the alloy. Alloys may age "naturally" meaning that the precipitates form at room temperature, or they may age "artificially" when precipitates only form at elevated temperatures. In some applications, naturally aging alloys may be stored in a freezer to prevent hardening until after further operations - assembly of rivets, for example, may be easier with a softer part.
Thermal stress
relieving:
- Stress relieving is a technique to remove or reduce the internal stresses created in a metal. These stresses may be caused in a number of ways, ranging from cold working to non-uniform cooling. Stress relieving is usually accomplished by heating a metal below the lower critical temperature and then cooling uniformly.
STRESSES
- When welding process is done heat is generated which is not applied uniformly. While welding, some part of the metal raises to very high temperature where the region adjacent to high temperature region remains at lower temperature. This results in different forms of expansion of material in different region. The region where it is supposed to be welded has very high temperature and the adjacent regions which has low temperature during this time the expansion of the material in higher temperature region is restricted by the region with lower temperature.
- The metals whenever it is melted in a small zone shrinkage stresses are created. For example if a bar is externally restrained during the time of heating and cooling process it contains some amount of stress in it. Theses stresses are referred to as residual stress which keeps the bar in the bent shape. When the bar is cooled down it cannot be bent furthermore and are stronger than forces exerted by residual stress. These residual stresses can be released through various methods. It can be done thermally by heating the whole metal uniformly for a particular time period. These are generally done below the transformation temperature of 1333ºF. By gradually increasing the temperature the stresses are allowed to relax and the metal recovers. Then the metal is cooled gradually after the stress relief, which produces the part with much lesser residual stress. This can help in avoiding the problems like distortion.
- There is some other methods also to relieve the stresses in the metal. One method is by giving vibratory treatment where the sound waves are passed and the residual stresses a relieved. This method have been effective in many applications.
- The other method is known as peening. In this method heavy pneumatic hammers are used to hammer the welded surface. This hammering action tends to deform the structure and reduces the thickness. This deformation tends to relief the residual stresses. When the peening is heavy care must be taken to prevent the weld from cracking.
DIFFUSION
- Generally the atoms in liquid state can move freely with respect to each other and change their respective positions. But even under certain conditions the atoms present in the solid also changes its position. They move away step by step from the home position and shifts its location in the solid structure. This change of position of atom in solid state is known as Diffusion.
- An example is let us take two flat bars of material gold and lead. These flat bars are tightly clamped together and at room temperature and let for several days. Later when the clamp is removed both the materials are found to be attached with each other. The reason behind this is the movement of atom between both the bars and resulting is diffusion which causes a weak metallurgical bond. Since the metallurgical bond is weak they can be broken easily. If the temperature of both the bars are increased the amount of diffusion increases and when the temperature is reaches above the melting point the material starts mixing with each other.
SOLID SOLUBILITY
- In General solid solubility refers to solubility of solids into liquids. And most of the people are not aware of solid getting dissolved into another solid like the example which was mentioned above lead solid getting dissolved into gold. For example let’s put some sugar into water and get it dissolved. When less amount of sugar and more amount of water is used the sugar gets dissolved so easily and turns into liquid when the amount of sugar is increased and the level of water is decreased then the sugar stops dissolving into water. This is called critical solubility limit. When the level of water is increased the sugar again starts dissolving.
- Metals also have same behavior and dissolving occurs when both are solid. Similar to the liquid and sugar the solids are have critical solubility limit which depends on the temperature. If the temperature is more the solubility and diffusion of the solid metal will be more.
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