The term steel does not mean any one single composition. It is a vast variety of alloys that can all be called steel. If the metal has only carbon, and does not contain any other intentionally added alloy (except the impurities), then it is called plain carbon steel. Sometimes, elements such as vanadium, chromium, molybdenum, nickel are added to improve the properties of steel. Such steels are called alloyed steel.
Steel Types
Plain carbon steel can be categorized into three: low carbon steel (containing carbon content less than 0.25%) ,medium carbon steel (containing carbon between 0.25% to 0.55%), and high carbon steel (containing carbon > 0.55%). As the carbon content increases, the difficulty in welding of plain carbon steel increases as well. Likewise, as the alloying content increases, so does the difficulty in welding that alloyed steel.
Most steels in commercial use can be satisfactorily welded, provided correct welding procedures are followed. In this article, we shall discuss in general a few aspects of oxyacetylene welding of steel.
Generally speaking, mild steel (or low carbon steel) is the easiest steel to weld. However, use of proper procedures cannot be abandoned for mild steel too, as problems associated with distortion can play havoc with mild steel as well. Such distortion can be countered by presetting the job where possible, sequential welding, slow welding, and a judicious use of restraints/ clamps. Use of these measures limits the distortion. It is difficult to eliminate distortion altogether.
Basically, the expansion and contraction of the base metal and weld metal must be understood and taken into account for any sound welding strategy. When steels are welded with oxyacetylene welding process, particular care must be taken to ensure that the flame is kept to neutral at all times.
OxyFuel Welding Process: General
In the oxyfuel welding of steel, the welding torch heats the metal and creates a weld puddle on the base metal. A welding filler rod is dipped into the puddle; this enables deposition of weld metal. Sometimes the fusion can be achieved without use of a filler rod as well.
The uniform and smooth feeding of the torch, and at the right location in the puddle is important to get a good weld. It takes skill on the part of the welder to not just melt the base metal and add filler metal, but also obtain fusion of the freshly deposited weld metal with the side walls of the work-piece. Especially, at the root of a joint, the flame should be directly correctly such that complete fusion of the root is obtained, while avoiding excess penetration at the same time.
Care must be taken that one does not end up overheating the weld with the flame. Doing so weakens the joint.
General Principles in OxyFuel Welding of Steel
For welding of most steels, a neutral flame type is used. The oxidizing type of flame is avoided. In order to ensure that the flame is not oxidizing, a slightly reducing flame is used, which is characterized by a small acetylene feather in the flame. This is used for high carbon steel, chromium steel, and alloys containing intentionally added nickel. By using a carburizing flame, higher welding speeds are possible.
High gas pressure should be avoided. This makes controlling the molten pool of weld metal difficult. High pressure makes for a harsh flame. The gas pressure and tip size should be just sufficient to produce a reasonable sized weld puddle and a full penetration of the joint.
The size of the weld puddle should remain uniform as the welding progresses down the length of the joint. Excess penetration should be avoided.
The inner cone tip of the oxy-fuel flame should not come into contact with the weld metal or the base metal. The tip is rich with carbon, and carbon must not have a chance to enter the weld metal by any chance. The hot molten metal is prevented from oxidation by atmospheric gases by the outer portion of the flame. The flame should be manipulated to achieve this.
Also, in order to protect the freshly melted metal from the tip of the filler rod, the filler rod should be dipped in the weld puddle inside the cover of outer flame or the envelope of the flame. It is not advisable to have the rod melt near the upper regions of the flame, and drip into the molten metal.
The filler rod used is generally of the same composition as the base metal. If a quality weld joint is desired, the filler rod should be of high quality, sourced from a reputed supplier.
Other Practical Aspects Of Welding Steel With Oxy-fuel Welding
Let us imagine that a butt weld between two mild steel plates is required to be made with oxy-acetylene welding. When the welding is begun, one must not aim to achieve fusion right away. This is a mistake. Instead, the torch should be rotated around in circles of decreasing diameter over the spot where welding is intended to be begun. When this is done, the acetylene cone should be just a little away from the work-piece surface.
When the surface becomes red hot in a radius of about three times the metal thickness, then is the right time to bring down the torch such that the acetylene cone just touches the metal surface. Here, particular care must be taken to ensure that the cone should just touch the surface. The tip of the torch must not touch the metal.
Remember that steel is the only metal on which the welding is done by having the cone touch the metal surface. With all other metals, the cone remains well away from the surface.
When a weld puddle forms at the bottom of the groove, the tip of the filler rod should be dipped in the puddle such that the tip melts. The torch should be weaved around in circles such that the metal fuses with both sides of the groove. Achieving full penetration requires skill and practice. As we finish the circular motion, the flame should be directed to melt the adjacent spot.
Steel, unlike cast iron, solidifies almost immediately the instant when flame is removed. This allows steel to be welded in vertical and overhead positions as well.
The welding on steel with oxy-acetylene process is a smooth operation if a proper tip size has been chosen and the right flame type is maintained. An oxidizing flame should be avoided in oxy-acetylene welding of steel. Doing so would create excess sparks. The tip size should be appropriate to the thickness of the base metal.
If the tip size is too large, the size of weld puddle produced is large as well, which is difficult to control and manipulate. If the tip size is too small, it would be difficult to properly melt the filler rod and fuse it with the side walls. In this scenario, the filler rod would get frequently stuck in the puddle. When this happens, one should not try to pull the rod out with force.
Instead heat should be applied on the spot such that the rod melts, and operation should be continued further.
It is necessary for the welder to be quite familiar with the process. He must have had adequate practice before attempting to weld on an application that requires high weld quality.
Sometimes, some welders try to twist some pieces of filler wires together so that the rate of weld metal deposition would be high. This is not advisable.
When unequal thickness base metals are welded, care must be given such that a higher amount of heat is directed towards the thicker metal.
Impurities When Oxyfuel Welding Steel
Oxygen, nitrogen and carbon are impurities that cause a weld metal to become defective. Oxygen and nitrogen give rise to increased porosity, blowholes, oxides, and slag inclusions.
The atmospheric oxygen combines with hot metal to produce oxides which render the weld porous and brittle. Due to this reason, oxygen is undesirable in the weld metal. The oxygen may also occur if an oxidizing flame is used. The oxygen causes foaming at the molten weld metal, and gives off sparks.
The atmospheric nitrogen combines with hot metal to form metal nitrides in the weld metal. The nitrides are hard structures, and undesirable in the weld.
Excess carbon gets introduced in the weld metal when a carburizing flame is used. Excess carbon is highly undesirable, as it renders the microstructure hard and brittle.
However, through proper manipulation of the welding torch and the filler rod, these oxides formed by the oxygen with the hot weld metal can be removed. The oxides that are deposited at the surface are not harmful though; these can be removed easily after welding, by using a wire-brush.
Edge Preparation For OxyFuel Welding Of Steel
When plates of thickness up to 3/16 inch (4.8 mm) thickness are to be welded, no edge preparation is generally necessary. A plain square butt joint is alright. However, a root gap equalling the thickness of the plate should be kept, for ensuring complete penetration of the root. Sheet metal, is however, prone to warping and distortion. Adequate allowance should be made for this.
Such low thicknesses can also be welded without using a filler rod as well. However, the strength of such welds is low in comparison with the welds that are made with the use of a filler rod. Sometimes the edges of such low thickness plates can be upset. The upset edges then take the place of a filler rod.
When the plates to be joined are more than 3/16 inch (4.8 mm) thick, a bevelling of the plates becomes quite necessary, and cannot be avoided. This is necessary to obtain complete fusion at the root. Forehand welding technique may be used.
When the thickness of plates to be joined is of the order of ½ inch (12.8 mm) to ¾ inch (19 mm) thick, a U-groove type of edge preparation is recommended. A root face (also called ‘land’ in some places) is used to cushion the first layer of weld metal. The backhand type of welding technique is used for these thicknesses.
At higher thicknesses, a double-V or double-U type of edge preparation is recommended, if access from both sides of the joint is available. If, however, access is available only from one side, a single-V or single-U would have to do.
So this was about oxy-acetylene welding of steel. Please share your thoughts in the comments section below.
Read More: New to oxy-fuel welding process? Read this article to get an introduction about the principles it operates on, gases used in its operation, and the equipment used in the process.