Geometric defects-undercut-excess reinforcement-distortion, Mechanical Engineering

Geometric Defects

 

Undercut

Undercut represents a depression into the base metal adjacent to the toe of the weld and if it forms a notch, it is a potential danger for the weld. A small amount of undercut under pulsating stress often acts as a starting point for fatigue cracks. When fatigue testing the welded joints under pulsating tensile stress, cracks very often start from slight undercutting at the edge of the weld and propagate through the base material. The fatigue strength obtained when testing unmachined welded joints is therefore a measure of the notch sensitivity of the base material rather than the fatigue strength of the weld metal. High tensile steels with a high yield strength I tensile strength ratio are much more sensitive to notches than unalloyed steels as regards to fatigue strength.

Fatigue tests on notched base material test pieces have shown that unalloyed C steels with a tensile strength of about 50 Kg / cm2 sustain some 3 or 4 times the number of cycles than low alloyed steels with double the static tensile strength. It must be understood that the high tensile strength of the high tensile steels can only be ut ilised in welded construction subjected to dynamic loads, if the weld reinforcement and undercuts are removed. Correct choice of current range and good manipulation of electrode during welding will avoid undercut. Undercuts are to be dressed smooth without loss of thickness beyond parent metal thickness. Visual examination together with boroscope is usually suitable for the observation of undercut in tubular welds.

Excess reinforcement

Weld reinforcement represents the metal on the face of a weld in excess of that necessary for the specified weld sizes. The height, width and radius of the bead curvature are important in a weld joint. The reinforcement angle is the primary parameter affecting fatigue strength. Fatigue tests have shown that the shape of the weld reinforcement can affect the fatigue strength. Necessary care must be taken during finish run to give correct reinforcement for the weld. The dressing of weld also ensures this.

Burn through or excess penetration

It is found at the root of the weld. Sometimes this creates an opening at the root. This is a serious defect in Tabular welds. This occurs because of the usage of very high current and low travel speed. The net result of excess penetration in tubular welds is the reduction in bore size. Also, excess penetration affects the fatigue strength of the material considerably as sharp notches are formed by the weld metal. Normally, ball test and radiography are made use of to ascertaintheir presence. Use of consumable inserts restrict the excess penetration.

Distortion

Distortion is the deviation from the desired form or shape and is due to unequal heating and cooling which causes unequal expansion and contraction. During welding, the heat input by the energy source causes the melting of the metal at one point and results in severe temperature gradient along with a variation in the mechanical and chemical properties. Depending upon the change in properties, the consequent thickness and weld configuration, the amount of distortion varies from one weldment to another. The resistance created by the surrounding cold metal to the cooling weld metal is the cause for residual stresses and distortion.Distortion depends upon

  • The magnitude of stress developed
  • The distribution of these stresses in weldments
  • The strength of members upon which it acts
  • The restraint on the assembly.

 


The effect of distortion is mainlydimensional. This can affect assembly and the performance during service. Distortion can be controlled by

  • Design
  • Accuracy of manufacture
  • Assembly procedure
  • Counter allowance and
  • Welding procedure
Posted Date: 9/17/2012 9:23:53 AM | Location : United States







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