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Metallurgical changes in Weld and Heat Affected Zone

 Metallurgical changes in Weld and Heat Affected Zone

This blog focuses on the metallurgical changes in the weld and heat affected zone of steels. We blog describes the structural changes in the weld and heat affected zone, particularly in the case of carbon steel welded joints.

Carbon Steel Categories

Carbon steel is categorized based on carbon content, with residual elements such as manganese, silicon, sulfur, and phosphorus not expected to significantly affect the metallurgical and structural properties of the steels. The categories are:

  • Low carbon steel: up to 0.15% carbon content
  • Medium carbon steel: 0.15 to 0.5% carbon content
  • High carbon steel: greater than 0.5% carbon content

In general, an increase in carbon content increases the tensile strength and yield strength of carbon steels but decreases ductility and toughness. The changing proportion of the ferrite and pearlite phases formed attribute to this variation.

Metallurgical Properties and Phases

The mechanical properties of carbon steel change significantly with the changing proportion of phases formed. With an increase in carbon content, there is a continuous increase in hardness and strength up to the eutectoid point, beyond which there is a continuous decrease in strength and ductility. The formation of the cementite phase along the boundary of the pearlite also contributes to the decrease in strength.

In welding, the kind of phases formed will depend on the weld thermal cycle experienced by the different zones. The point in the weld zone experiences a temperature higher than the liquidus temperature, and the different points experience varying temperatures based on their distance from the fusion boundary. Under equilibrium conditions, the base metal experiences a continuous change in structure from ferrite plus pearlite, alpha plus austenite plus austenite fine, austenite to coarse austenite.

Weld Thermal Cycle and Heat Treatment Cycles

The weld thermal cycle and heat treatment cycles differ in terms of the way temperature variation occurs as a function of time. The weld thermal cycle experiences a higher heating rate followed by rapid cooling, while the heat treatment cycle experiences a lower heating rate followed by controlled cooling. Exposure to high temperature is for a much longer period in the weld thermal cycle, and the rate of heating and cooling are slower compared to the heat treatment cycle.

Introduction

During welding, heat treatment is performed in the austenitic state of most steels, which results in high temperatures reaching up to the fusion temperature. However, the heat treatment is carried out normally in the austenitic temperature band which is like say 50-degree centigrade above the upper critical temperature limits.

The limited availability of the high temperature brings in the lot of heterogeneity in terms of the structure which is formed and that will be leading to the lot of structural variation and heterogeneity in the properties of the weld joint.

Structural Transformations

Continuous change in this structure here martensite but the coarse martensite then fine-grained zone will have fine pearlite and the partially refined zone will have the partially fine pearlite so there will continuous change in terms of the phases as well as in terms of the grain structure or the size of the grains so these are the typical structural transformation which will be occurring in case of the low carbon steels.

If very heterogeneous austenite is formed where due to the lack of time the pearlite which is of higher carbon content during the subsequent cooling phase of the welding high carbon austenite formed as a result of the transformation from pearlite to the austenite so this will be resulting in the high carbon martensite also.

In general, if we plot the variation in the hardness as a function of the distance from the fusion boundary, then typically in the weld joints in the simple carbon steels, the variation like this is a fusion boundary what we will see that as a function of the distance from the fusion boundary we say this significant higher hardness and then hardness will keep on decreasing so this high hardness is attributed to the martensitic transformation then finer fine-grained pearlite then partiality fine pearlite and then the base metal so these are the three different zones which are formed coarse martensite offering much higher hardness as compared to the other areas.

Micrographs

The weld zone will be heated above the liquidus temperature and then the next to the fusion boundary there will be the two-phase zone which will be falling between the liquidus and solidus and then will be having the heat-affected zone that is below the solidus and above the upper critical temperature zone and then we will have the base metal and the base metal.

Structural Changes in Carbon Steels during Welding

During welding, the temperature will be below the lower critical temperature, resulting in the formation of different zones. The relation with the iron-carbon diagram shows a significant difference in the weld thermal cycle and the heat treatment cycles used in the two cases.

  • The weld fusion zone experiences a temperature above the liquidus
  • The heat-affected zone experiences higher temperatures for a longer period
  • Point D experiences the coarse grain coarsening and the formation of austenite
  • Point C experiences fine grain refinement, resulting in fine pearlite
  • Point B is in the two-phase zone, experiencing a partially refined zone
  • Point A is in the base metal, which has different phases



The base metal corresponding to the 0.3% carbon steel at points 1 and 3 will have ferrite and pearlite. When heated to point 2, the pearlite transforms into austenite, while the alpha remains as it is. Rapid cooling results in partially refined zones.

Point 3 is located just above the upper critical temperature, and all the austenite, ferrite, and pearlite transform into fine austenite, promoting fine pearlitic grain structure. Heating point D to a much higher temperature for a longer period results in grain coarsening and the formation of coarse austenite, promoting martensitic transformation.

The heterogeneity of the weld thermal cycle results in changes in the structure ranging from ferrite-pearlite in the base metal to fully refined pearlite in the refined zone. High carbon steels invariably form martensite in the heat-affected zone, leading to embrittlement and loss of toughness. Therefore, tempering of the weld joints is done to reduce residual stresses and induce toughness.

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