This article discusses about welding of medium carbon steel. It offers a few pieces of advice for obtaining a quality weld.
Introduction
The carbon content in medium carbon steel ranges from 0.30 to 0.55 percent. After welding, these steels are heat treated to relieve the residual stresses.
Medium carbon steel is fabricated in the bar form in cold-rolled or normalized and annealed condition. The strength of this steel is higher than low carbon steel owing to a higher carbon content. In lieu of high strength, ductility is lower.
Welding Of Medium Carbon Steel
For these steels, a preheat of 300°F( 150°C) to 500°F (260°C) is generally necessary. As the carbon content increases, preheat becomes further necessary. Preheat also depends on the section thickness of the part to be welded. Any thickness below ¾ inch (19 mm) needs no preheat.
Above 19 mm, it is better to use a preheat of at least 200°F (95°C). Preheat temperature can be checked by using a thermal chalk that comes specifically designed for this purpose. The chalk is rubbed on the preheated surface. If the part has reached the desired temperature, the chalk melts and evaporates. Else, it doesn’t.
Preheat is necessary to slow down the rate of cooling in these steels. Overall, the idea is to keep the rate of cooling under control. Else, high rate of cooling results in a hardened structure in heat affected zone.
Most of the usual welding processes can be used to weld medium carbon steels, such as arc welding, gas welding, resistance welding, and so on.
When welding steels containing small amounts of alloying elements such as manganese, and especially in high base metal thicknesses – the susceptibility of the weld to cold cracking is high. So, other ways of minimizing hydrogen in the weld pool should be explored.
Such as, electrodes that come with a low diffusible hydrogen rating should be used. These electrodes come with a designator of H2 or H4 or H8 or H16.
‘H’ indicates hydrogen, and the numeral indicates the amount of hydrogen that can be expected in weld metal deposited with that electrode, in ml/100 gms. For example, an electrode with a ‘H4’ diffusible hydrogen designator would deposit weld metal containing less than 4 ml/100 grams of metal.
In a manner similar to high carbon steels, sometimes these steels too are ‘softened-up’ by annealing. This makes them amenable to welding. After welding, the whole piece is subject to an appropriate heat treatment to restore the original mechanical properties.
Preheat And PWHT
Welding of medium carbon steel requires preheat. Applying preheat slows down the rate of cooling. A slow cooling rate keeps the microstructure from turning to hard martensite which is brittle and crack-prone.
After welding, the joint should be subjected to a stress-relieving post weld heat treatment (PWHT). The temperature of this PWHT varies between 1100°F to 1250°F (593° to 677°C), and the time of soaking depends on the thickness. As a thumb rule, one hour of soaking time per inch (25 mm) of the section thickness is given to the weld.
During the cooling from soaking temperature to room temperature, it is important to keep the rate of cooling to low values. In modern furnaces, this can be achieved by feeding the value of desired rate of cooling in the computer program. In local heat treatment, rate of cooling is slowed down by covering the parts with a fire-resistant material.
Things To Keep In Mind In Welding Medium Carbon Steel
When welding carbon steels containing medium carbon, the following guidelines should be kept in mind:
The edge preparation in welding of medium carbon steel is similar to that used in low carbon steel.
SMAW electrodes with low carbon content should be used. The welding heat should be properly controlled such that overheating of the weld metal does not occur. Overheated weld metal results in excessive sidewall penetration, which brings more carbon from base metal into the weld metal, through dilution.
Such control of heat is achieved by directing the electrode towards the already deposited bead, instead of directly on the sidewalls. This causes the molten metal to wash up to the side walls and fuse with the base metal, without causing deep penetration.
High heat input should be avoided in medium carbon steels. Because, high welding heat causes fusion zone adjacent to the weld metal to become hard and brittle, and therefore susceptible to cracking. Such hard structure in heat affected zone can be avoided by making small stringer beads in a series of passes.
Each bead of weld metal ‘tempers’ the weld metal just below it. The grain size of the layer immediately below gets refined by the annealing effect produced by depositing the new bead.
Post-weld heat treatment of a completed weld, as discussed above, is necessary to relieve the residual stresses in the weld. The rate of cooling should be kept low.
When stainless steel electrodes are used to weld on medium carbon steels, string beads should be deployed. This helps in avoiding dilution from the carbon rich base metal. Carbon is not desired in a stainless steel microstructure because it promotes carbide precipitation.
In high thickness base metals, weaving beads can be deployed in the upper layers of the section. The weaving motion in these beads should not exceed three times the diameter of electrode being used.
Like all welding, cleaning of deposited beads is essential to obtain a quality weld. Using chipping tool, and a wire-brush – the deposited weld metal should be thoroughly cleaned to remove any traces of slag before depositing next pass.
Welding With Oxyfuel Welding Process
A filler rod with low carbon content should be used. Rods made for these steels have small amounts of alloying content that make up for the deficiency in strength due to low carbon.
A slightly carburizing flame flame should be employed. The welder should aim to keep a small puddle size, to obtain a quality weld. A carburizing flame permits welding at high speed.
Can Medium Carbon Steels Be Brazed?
Yes, medium carbon steels can be brazed. A preheat of 200°F to 400°C (95°C to 200°C) should be used, and a brazing flux can be used for shielding. However strength of such joints is not good as compared to welded joints. Hence, welding is the preferred method of joining these steels, especially arc welding.
So this was a little about welding of medium carbon steel. Please share your thoughts in the comments section below.