Good Welds and how to make them .

                         GOOD WELDS AND HOW TO MAKE THEM

Now that the major metallurgical considerations in welding have been discussed, the reasons of why welds are preferred in joining metals will be better understood.

Welding is the only commercial method which can be used for joining metals that is capable of giving full strength to the joint. Welds having yield strength and ultimate tensile strength equal to or greater than the parent metal, are usual rather than the exception. Filler metals in welding rods and electrodes are carefully engineered, their chemical composition is rigidly controlled, and their melting and deposition by electric arc assures higher quality than is normally available in the parent metal .

                           Comparative Properties of Weld Metal

                

	ASTM-ASME PLATE METAL
PROPERTIES	A-7 (MILD STEEL)	A-242 (LOW ALLOY)	WELD METAL DEPOSITED WITH E-60XX ELECTRODE
UIT. Tensile Strength, psi Yield Point, psi Elongation, % in 2 in.	60-72,000 33,000 28	65-75,000 50-65,000 20-22	62-90,000 52-86,000 17-25

Tensile strength and toughness are the two most important properties in evaluating how well a joint will perform .

As deposited, the weld metal should have higher yield strength than the parent metal. Thus, any accidental overload of the structure or assembly will produce yield in the parent metal rather than in the weld. Since the parent metal has much greater mass than the weld, failure is less likely to occur.

The higher tensile strength of the weld metal has no adverse effect on the pieces joined by it. Below the yield point, the welded members act as a continuous beam. Table shown above compares typical properties of mild steel weld metal (E-60xx electrode) with those of two common structural materials-one a mild steel, the other a low-alloy high-strength steel.

The relative ductility of materials, as measured by standard elongation tests, is not ordinarily of any great significance. No more than a few percent elongation is made use of during the usual service of a weldment. No appreciable elongation can occur until the yield point of the material has been reached. Since the structure is designed with a large safety factor, the stress is not at all likely to reach the yield point. Thus, comparisons are of little value. If the weld metal has a higher yield point than the parent metal, the possibility of its full ductility ever being needed is very slim.

When materials of very high strength but little ductility are joined in a structure, it is often desirable to weld with an electrode that will furnish weld metal of good ductility so as to minimize shrinkage stresses. But most welding is not in this category.

For lack of a better yardstick, ductility (elongation) is often asked for when toughness is the property actually desired. Toughness is not easily defined, but it is the ability to absorb energy, not only the ability to deform. This toughness is partially the result of ductility, and also is related to tensile strength.

The usual values for tensile strength and elongation are obtained from tests made at room temperature under a slow pulling load. Two metals that test the same for tensile strength and elongation may vary appreciably as the velocity of the applied load increases.

Standard impact tests provide values that represent the metal's toughness under impact but more truthfully reflect the metal's notch sensitivity. The values change radically under different types of impact, and as the size and shape of notches change.

Most failures from shock loading are related to metal fatigue, or occur at low operating temperature. Toughness evaluated at room temperature may be lost at a lower temperature. The important factor in early shock-load failures is the transition temperature of the metal that point below which the metal fails under shock loads by brittle fracture with little or no apparent absorption of energy.

Properly designed weldments normally will not fail from shock loads, provided the transition temperature of parent metal and weld is lower than the temperature at which the weldment will operate. Where weldments are to be used outdoors or otherwise exposed to low temperatures, electrodes having low transition temperatures are desirable.

Electrode types in order of desirable transition temperatures are as follows: low-hydrogen types E-xx16 or E-xx18 (lowest transition temperature); E-xx24 and E-xx27; E-xx10 and E-xx11; E-xx13; and E-xx12 (gradually increasing to highest transition temperature).

In many cases, the fact that mild steel welds made by metal-arc welding are usually stronger than the base metal is overlooked. The tendency is to seek insurance against weld failures by building up the weld thickness. Where codes exist that cover weld specifications these provide a safety margin. The designer calculates the required weld size and then adds a bit for good measure. The weld shop foreman looks at the print and decides he'd better increase the specified size to be safe. The job then goes to the weldor who, not knowing the chain of additions, makes the prescribed weld on the heavy side.

In the case of a fillet weld, the ⅟₄ inch weld that becomes ⅜ inch takes twice as much filler metal!-unnecessarily. And, the over welding may cause distortion and high locked-in stresses that actually reduce the effective yield strength.

Butt welds, too, are often over welded. Pic below  shows a set of welded tensile specimens, not one of which was welded all the way into the root. Two of these welds were reinforced by build-up, and the others tested after being ground flush with the surface. Each of these test bars failed through the parent metal and not through the weld or the heat-affected zone. 100% penetration should not be specified unless loading demands it.

Welds are stronger than most people realize. Even porosity, undercuts, slag inclusions, and other "defects" have far less influence than is commonly acknowledged. Of course, some jobs must have more rigid control of these defects than others. A rotating shaft, for instance, when ground after welding will have its fatigue strength much reduced by the presence of even minute pores in the weld deposit. Titanium used in a highly stressed wing structure must not be porous.

In Pic above  - All Four butt-welded tensile bars failed in the parent metal even though weld penetration was incomplete in each case. Root opening in specimen at left is over 30% of effective throat; yet the weld held

The weldor can't be expected to investigate all the conditions under which the weldment must give service. A good weld, then, is one which meets the job specifications when they exist. Such specifications usually govern production welding and field construction, and often do not exist in repair welding.

Every Weld a Good Weld

Every weld should be a good weld. Occasionally something goes wrong. The most frequent cause of a poor weld is carelessness on the part of the weldor, generally involving a poor welding technique. "Any job worth doing is worth doing well." This saying is particularly applicable in welding.

Doing a job well does not mean over welding. It means doing the job properly, depositing sufficient weld metal to meet the structural requirement of the job but not so much as to make it too costly. Care and pride in the work being done would help eliminate some of the defective welds produced.

The weldor should remember that properly made welds will result in products that are stronger, lighter in weight, more pleasing in appearance and generally at less cost. His continued success depends on his making good welds. If careless habits or the lack of knowledge causes him to make poor welds, these faults must be corrected.

The best means of overcoming such difficulties is to determine what causes the defect and how it may be corrected. That is how good welds may be made consistently.

The Good Weld

It is the aim of every weldor to produce good welds. A weld is not satisfactory if it will not perform the function desired of it. Many characteristics are expected of a satisfactory weld . It should be strong , yet have sufficient ductility to withstand the service conditions to which it will be subjected. Usually, a weld is expected to be sufficiently tight to provide a leak proof joint. In some instances. This leak proof characteristic is expected to resist corrosion as well. Wear resistance, likewise, is a quality that is important, so far as welding is concerned. Appearance, likewise, plays an important role in the finished product. In fact, a satisfactory weld usually produces a product that is lighter in weight, stronger, more pleasing in appearance and generally less costly.

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Reference : 
The James F Lincoln arc welding foundation .