TECHNICAL PILLS: WELDING DEFECTS

INTRODUCTION

The welding process was briefly introduced in the previous article, indicating the most used methods in the industrial field, highlighting advantages and disadvantages.

The welds can be characterized by a series of imperfections and/or defects that could compromise the mechanical strength of the joint. In particular, the final characteristics of the weld are influenced by three aspects:

• Operational Imperfections: they are related to the operative skills of the welder in relation to the type of the joint and to the welding position;
• Metallurgical Imperfections: they are related to the metallurgy of the weld, directly depending to welding parameters in relation to the welded joint;
• Mechanical characteristics: the welded joint must fulfill specified mechanical properties such as strength, hardness, toughness, etc.

WELDED JOINT

It is possible to distinguish three zones at a welded joint:

• Base Material (BM): it is the material that must be joined. During the welding process it does not undergo to such heating as to create microstructural changes;
• Heat Affacted Zone (HAZ): consisting of the base material which, although not having reached the melting temperature, has been subjected to a heating and cooling cycle such as to cause transformations in the crystalline structure;
• Fusion Zone (FZ): composed of the material that is melted during the welding process and solidifying gave rise to the weld bead.

 

WELDED JOINT DEFECTS

The ISO 6520-1:2008 standard defines as imperfections any modification with respect to an ideal welds and defects as a not acceptable imperfection. The same standard divides the imperfections in 6 groups:

• CRACKS
• CAVITY
• SOLID SLAG INCLUSION
• LACK OF FUSION OR INCOMPLETE PENETRATION
• SHAPE AND DIMENSIONAL DEFECTS
• OTHER IMPERFECTIONS

CRACKS

This is the most dangerous weld defect as it is a possible high stress concentration zone. The cracks could propagate and lead to breakages.

Craks are detachments between two edges of materials, with bidimensional geometry. They can be found both in the FZ and in the HAZ.

They can be classified as:

• LONGITUDINAL CRACK
  • Longitudinal crack in the FZ: typically, hot cracks;
  • Longitudinal crack along the fusion line: generally due to the operating conditions and to the metallurgical inhomogeneity between the FZ and HAZ;
  • Longitudinal crack in the HAZ: they can be cold crack or heat treatment crack;
  • Longitudinal crack in the base material: lamellar tears or due to the operating conditions;
• TRANSVERSAL CRACK
  • • Transversal crack in the FZ: generally cold crack;
    • Transversal crack in the HAZ;

  Transversal crack in the BM;

   

• CRATER CRACKIt forms near the end of a weld section. The shrinkage of the metal during the cooling phase causes the formation of a crater due to the suction of the still liquid metal from the surrounding areas.

 

 

 

 

Considering the origin of the crack formation, it is possible to distinguish:

• HOT CRACK
  They originates in almost all metals (ferritic, austenitic, nickel alloys, aluminum alloys) during the solidification of the metal. They generally have longitudinal orientation and location in the FZ in the weld bead point that cool down more slowly (the central part). The main cause is due to the presence of low melting compounds, which, during the solidification of the surrounding metal, can’t fulfill the shrinkage stress. Low melting compounds form due to the presence of dirt and/or humidity in the edges of the base material or in the filling material, but also for the presence of carbon, sulfur, phosphorus in the base material that delay the solidification of the intergranular zones.
• COLD CRACK
  These cracks originates when the weld is completely cooled. They are generally oriented in the transversal direction of the weld. They originate in steel with ferritic or martensitic matrix. The presence of hydrogen (humidity) in the weld pool leads to a significant drop of the material ductility. This, with the formation of fragile structures in the welded area, leads to the formation of crack in the fused zone if the filling material is more hardening than the base material or in the HAZ in the other way around.

 

CAVITY

It generally consists in the absence of material. The presence of cavity is generally due to traces of trapped gas and released during the fusion, to chemical reaction that occurs during the welding or to the presence of contaminating elements in the fusion pool.

INCLUSIONS

These are foreign substances of various kinds trapped in the molten metal.

The most common inclusions are:

• SLAG AND FLUX INCLUSIONS

It is one of the most common imperfections in beads made with coated electrodes (slag inclusions) or submerged arc (flux inclusions) in multiple passes. Their presence is essentially due to an inaccurate removal of slag between the execution of one pass and the next, incorrect handling of the coated electrode or incorrect positioning of the welding head, as well as incorrect preparation of the edges.

• METALLIC INCLUSIONS
  • Tungsten inclusions:
    It is a typical imperfection of the TIG process. Possible causes include incorrect handling of the torch, insufficient gas protection, too high current;
  • Copper inclusions:
    they are characteristics of welding on an infusible support. They can be responsible for a metal embrittlement or a reduction of the resistant section.

 

LACK OF FUSION AND PENETRATION

In general, these are situations in which:

– the electrical parameters are not correct (the current is too low),

– the welding speed is too high

– the operative technique is inadequate

– The preparation of the edges is not correct.

They are dangerous defects in consideration of the two-dimensional geometry of the imperfection it causes a local intensification of efforts (notch effect).

 

LACK OF FUSION

It is a lack of connection between the deposited metal and the base metal, or between contiguous layers of deposited metal.

They are typical of carbon or low-alloy steels, for processes such as oxyacetylene welding (i.e. with low heat input) or MAG (i.e. with low heat input).

Based on the position of the gluing, it is possible to distinguish lack of lateral fusion, lack of fusion between the passes or lack of fusion at the vertex.

 

LACK OF PENETRATION OR INCOMPLETE PENETRATION

This is a difference between the penetration required by the designer (nominal) and the actual penetration.

The main causes are:

– Poor preparation of the edges (opening angle of the groove too small, excessive shoulder, insufficient distance between the flaps, unevenness);

– Inadequate recovery

– Poor skill of the welder, in the case of non-automatic welding processes.

Generally, the defect is localized at the top of the weld (lack of penetration at the top), both in head-to-head, T-shaped and corner bead welds.

 

SHAPE AND DIMENSIONAL DEFECTS

These are profile imperfections on the external surfaces of the weld or the faulty geometric configuration of the joint.

Operational imperfections therefore belong to this group.

The main imperfections that fall into this group are:

– Excess weld metal: the weld bead is very high. It indicates a too high current, often it is not even considered a defect (indeed one could even believe that the resistance of the weld goes hand in hand with the size of the seam) but since irregular profiles give rise to areas of concentration of the stresses, it is absolutely necessarily making a suitable adjustment of the current intensity.

– Incomplete filling: Specular defect to the excess of weld metal, it is an indication of excessive penetration or insufficient supply of material.

– Undercut: They originate when there is an excessive melting of the base metal (therefore excessive current or incorrect positioning of the torch) and since they give rise to an irregular profile they can also be areas of concentration of efforts.

– Excess of penetration

– Overflow

– Breakthrough

OTHER IMPERFECTIONS

 

This group includes defects that cannot be included in the other groups.

The main ones are:

– Arc, grindstone, or chisel hits due to the operator’s inexperience which are often accompanied by more or less large cracks.

– Weld spatter, or deposits of molten material detached from the weld bead, absolutely to be avoided as they are possible sites of corrosive phenomena.

– Color defects, types of metals susceptible to oxidation and indices of inadequate purity of the gas.

 

CONCLUSIONS

The main types of imperfections that can be found in welding have been shown. Considering the danger of an imperfection, and therefore in establishing a criterion of acceptability of the same (possibly depending on its size), it is necessary to consider the context in which this imperfection is found.

Given the problems and its consequent impact on the properties of the materials that make up a construction, welding has posed, more than any other production process in the workshop or on site, the need to carry out accurate checks of all the preventive conditions of the materials, techniques and human resources used.

In the next article we will talk about welding quality control, which includes, in addition to the actual control of the finished weld, preliminary tests and checks, as well as partial checks during construction.