As the excessive as well the insufficient heat lead to complex changes in the microstructure as well the grain structure of the base material due to which the materials property were affected. How to overcome this difficulty?? All suggestions and open discussions are welcome.
The process of grain growth is a complex one for which you may find some detailed info here : https://web.wpi.edu/Pubs/E-project/Available/E-project-042211-104204/unrestricted/MQP_-_Grain_Growth_Kinetics_in_Steels.pdf
Grain size is well known to be in direct relation with mechanical properties of a steel. In general we can say that the bigger the size, the lower the ductility.
In welding, a part of the heat affected zone nearby the weld, will be affected by the heat genarated during the welding in the weld pool causing a small band aside the fusion zone where the base metal will reach during a short time, a temperature level able to start a grain modification called grain growth. The temperature level and the time spent at such temperature are main drivers to indicate what will be the intensity of the grain modification. In welding of materials, the energy communicated to the base metal can be estimated by a simple relation : the heat Input. To calculate the heat input, generally for welding processes using electrical heating, current Amps have to to multiplied with arc voltage and divided by the travel speed of the arc. This means that if you modify the values of these parameters, you will change the heat communicated to the metal being welded which on its turn will have an immediate influence of the temperature level reached in the heat affected zone (aside the molten metal) and the time during which this temperature will be maintained. Thus at the end it will have an influence of steel grain structure nearby the weld which is in direct relation with the temperature reached and the time spend at this level or above it.
More info here : Article Study of the sensitivity of high-strength cold-resistant shi...
Dear Dr. Tanvir Singh,
for many applications that involve thin plain carbon steel such as A36 or similar grades, these variables have minimal effect on the final welded joint. The variables become much more important when you are welding higher-strength or alloyed base materials, so we will focus on those applications.
To produce high-strength materials with good toughness properties, there are two types of material that have different mechanisms used in their manufacture: high-strength, low-alloy (HSLA) and thermo-mechanically controlled processed (TMCP) steel.
Thewelding heat input calculation is defined by voltage, amperage, and travel speed, and it is generally expressed as kilojoules per linear inch of weld (kJ/in.).
The general heat input range recommended for welding various types of carbon steel and specialty alloys is 30 to 70 kJ/in. However, not all materials are bound by that range.
Heat input from welding can have a serious effect on the mechanical toughness properties of the base material in the heat-affected zone (HAZ) and the weld metal itself. As a rule of thumb, the higher the strength and/or corrosion resistance of the base material, the higher level of precaution should be taken during welding.
In most cases, problems arise when the heat input is too low or too high. Heat input that is too high, either through excessive voltage, excessive amperage, or slow travel speed, can slow your cooling rate, resulting in excessive grain growth. This excessive grain growth shows up in changed mechanical properties, mainly a decrease in the material's cold weather toughness.
Using incorrect welding parameters such as excessive voltage can cause small amounts of alloy to be lost in the arc, directly reducing the effective mechanical properties of the welded joint. This is also true with stainless steel, the result being degradation in corrosion resistance. For stainless steel, restricting heat input to 50 kJ/in. or less should prevent this from happening.
Slow travel speed usually occurs when you attempt to fill a large joint with only a few weld passes. This raises the heat input drastically. The solidification rate of the weld pool occurs at a slower rate, promoting grain growth in the weld metal and HAZ. Large grain size tends to produce a weaker microstructure but is largely dependent on filler metal and base metal chemistry, percentage of dilution, and peak temperatures.
For more details, please see the source:
-Heat input and its effects on material properties by ESAB Knowledge Centre
Available at: https://www.esabna.com/us/en/education/blog/heat-input-and-its-effects-on-material-properties.cfm
Best regards, Pierluigi Traverso.
https://www.esabna.com/us/en/education/blog/heat-input-and-its-effects-on-material-properties.cfm
is important to avoid significant grain grow in welding process. in weld decreased with the heat input. grain coarsening is found and the grains even refined to improve the properties of the material. of the welded joint is found to be relatively coarser at high heat input.
Increases in the arc voltage and welding current resulted in increased hardness and decrease in yield strength, tensile strength and impact toughness. Increasing the welding speed from 40‐66.67 mm/min caused an increase in the hardness characteristic of the welded samples.
https://res.mdpi.com/d_attachment/metals/metals-10-00079/article_deploy/metals-10-00079.pdf
Microstructural evolution and mechanical properties of an ultra‐high strength structural steel welded joint at different heat inputs are investigated by metal active gas arc (MAG) welding technique. It is found that the microstructure of weld metal (WM) contained high volume fraction of acicular ferrite regardless of heat input. However, the acicular ferrite changes from bainite acicular ferrite (B‐AF) to Widmanstätten acicular ferrite (WF‐AF) with the increase in heat input. The microstructure of coarse grain heat affected zone (CGHAZ) is characterized by complete martensite, a mixture microstructure (lath martensite and bainite), and granular bainte, respectively, with increasing heat input from 7.5 to 10.5 and then to18.5 kJ cm−1. The acicular ferrite in WM nucleated at complex oxide inclusions and the number density of inclusions with the size of ≈0.6–1.2 µm significantly increases with the increase in heat input. The maximum drop of strength (113 MPa) occurs when the heat input is up to 18.5 kJ cm−1. High volume fraction of acicular ferrite in WM improves the impact toughness because of increased proportion of high angle grain boundaries and interlocking structure. The heat input should be higher than 7.5 kJ/cm to avoid the formation of welding cold crack and twinned martensite in CGHAZ.
Thank you very much for your valuable commentS and wonderful explanations. Jean-Pierre Van Nieuwenhoven and Pierluigi Traverso
Heat treatment is one of the choices used to recover the grain size and thermal stresses.
Check this paper
Improving the Mechanical Strength of Ductile Cast Iron Welded Joints Using Different Heat Treatments
Article Improving the Mechanical Strength of Ductile Cast Iron Welde...
Thank you for your valuable comment and such an informative discussion Abdulaziz Albannai
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