Brett Leary ME 296R Spring 2015

Weld Defects and Material Properties The causes, effects, prevention methods, and detection techniques for common weld defects, including inclusions, cracks, and incomplete fusion/penetration.

1 ABSTRACT

The causes, effects, prevention methods, and detection techniques for common weld defects,

including inclusions, cracks, and incomplete fusion/penetration, are researched. No perfect

materials or flawless weld processes exist and engineers must evaluate the effects that defects

have on their designs. Global sources from general weld experts, as well as aerospace and

marine applications, are consulted, with emphasis on aluminum and steel welded joints. The

works examined are focused on gas metal arc (GMA), gas tungsten arc (GTA), electron beam

(EB), laser, and friction welding processes. Detection techniques are concentrated on non-

destructive test (NDT) methods. Discussion includes purpose, findings, and conclusions.

1.1 KEYWORDS

Weld defects; Inclusions; Porosity; Slag; Cracks; Incomplete fusion; Incomplete penetration;

Effect on material properties; Weld defect detection

2 INTRODUCTION

2.1 MOTIVATION TO RESEARCH

Working at Tri-Tool Inc. as an engineering assistant, I was paired with the welder to gain

practical, hands-on education with welding, pipefitting, machining, and other manufacturing

processes. Mike worked all over the world on oil, gas, and nuclear construction projects and

taught me the lessons he learned in those environments. This was my first exposure to weld

processes.

My professional background today, lies in the aerospace industry at Aerojet Rocketdyne.

Welded joints are critical in many rocket designs, as they are in the oil, gas, and nuclear

construction businesses, though the materials are bit different. As quality engineer, I have a

reasonable understanding of how weld defects relate to material properties, I am by no means

an expert on the subject. I rely on weld, materials, and NDT engineers for their expertise. This

research affords me the opportunity to learn more about these topics.

2.2 RELEVANCE TO MATERIALS AND MECHANICAL ENGINEERING

In the absence of perfect materials and flawless weld processes, engineers must evaluate the

effects that defects have on their designs. Often these are dealt with in the design phase with a

safety factor intended to cover any anomalies or discontinuities, but for some products, that

just is not good enough. Weld defect allowance requirements are absolutely necessary to

maintain the integrity of certain tools, pipelines, wings, or pressure vessels. Failure of such

products presents a safety risk. Engineers may be trying to stretch the limits to maintain weight

or other requirements.

Understanding the causes, effects, prevention methods, and detection techniques for some of

the most common weld defects, helps engineers working to more strict design conditions. Not

simply for special cases, this knowledge aids engineers in selecting appropriate weld processes,

acceptable joint design, and interpretation of weld failures.

3 TYPES OF WELD DEFECTS

3.1 INCLUSIONS (POROSITY, SLAG, AND TUNGSTEN)

3.1.1 TYPES AND CAUSES

Porosity describes the gas holes or voids found within a weld, as shown in Figure 1Error!

Reference source not found.. These pores are caused by gases, which are lighter than the

molten metal, that do not escape from the weld as it cools and are trapped within the weld as it

solidifies. According to the American Weld Society, the root causes for porosity are dirt and

moisture on the base metal, in the electrodes and gases, and in the weld equipment [1]. The

oxide layer on the surfaces of aluminum alloys also contributes to porosity defects. The risk of

porosity is largest for manual metal arc weld processes with coated electrodes due to the

moisture in the coating and thicker oxide layers in the weld wire [2]. There are four main

categories of porosity: uniformly scattered, clustered, linear, and worm [1].

Slag describes oxides and non-metallic solids trapped within a weld. Slag forms from chemical

reactions that occur between the metal and the electrode, when not fully dissolved into the

molten metal. This slag can lead to more porosity within the weld [1].

For gas tungsten arc welding (GTAW), a non-consumable tungsten electrode creates the weld

arc between the tungsten and the part. Incidental contact between the electrode and the

molten metal may transfer tungsten particles into the weld.

3.1.2 EFFECTS

These inclusions negatively impact the material by acting as initiators for cracks that rapidly

lead to fatigue failures [5]. Pores nearest the surface of the weld are expected to initiate fatigue

cracks before pores further from the surface. “As this crack becomes the dominant singular ity,

no other cracks initiate” [3]. “Excessive concentration near one surface of the bead (most likely

the top) can impose a bending component which amplifies the stress” [4]. Pores can join

together to increase the stress concentration, from one pore to another, as the crack

propagates; see Figure 2. Clustered pores can grow together to behave like a single larger pore.

Increased total area of the pores relative to the weld cross section reduces strength; many

Figure 1 - Weld Porosity [1]

small pores have the same effect as a few large pores with the same total area [4]. According to

a marine weld study, “the single pore (is the) least detrimental in fatigue followed by co-linear

porosity, and uniform porosity. Cluster porosity is predicted to be most detrimental” [3]. In

static tensile tests, porosity affects “fillet welds far more adversely than butt welds,” reducing

tensile strength by up to 60% [2]. The yield strength of the material is insignificantly affected by

pores in the weld, while the ultimate tensile strength is reduced by increased porosity, as

shown in Figure 3Error! Reference source not found. [5]. “Ductility of the aluminium weld

metal is sharply reduced by the presence of small amounts of porosity” [5]. Slag and tungsten

inclusions are often evaluated in the same manner as the more common porosity defects [1].

According to Thielsch, static tensile and fatigue tests on aluminum alloys indicated that

specimens with substantial tungsten inclusions did not cause any failures of the joints [2].

Rework of inclusions can be a time consuming, expensive process. When the weld contains a

considerable number of pores with respect to the size of the weld, due to contamination, no

amount of rework may be able to correct them.

Figure 2 - Clustered Porosity Grows to Behave Like a Single Pore [2]

3.1.3 PREVENTION

Prevention of porosity depends somewhat on the weld process being utilized, but for all of

them, the cleanliness of the base metal, shielding gas, and filler metal are critical. Hydrogen

porosity can form from moisture in the air. Aluminum is especially susceptible to oxides on the

part surface, which must be etched or scraped to remove them. If the shielding gas flow rate is

too high or low, then porosity can result [5]. The filler metal must be stored properly to prevent

contamination by dust and dirt [7]. Store at the same temperature as the weld cell to prevent

condensation from any temperature change prior to weld [7]. The welder and fitter should

wear gloves to inhibit potential dirt, oils, and moisture from touching the part or filler wire [7].

Certain weld processes contain greater risk for porosity, slag, and tungsten than others. There is

a greater likelihood to find porosity and tungsten in a manual GTAW over an automated

electron beam or laser weld. Aluminum GTAWs often look like Swiss cheese when sectioned as

they contain many small pores; aluminum EB welds often have many fewer pores, but the

pores they do contain tend to be larger in size than the average pore in the aluminum GTAW.

3.1.4 DETECTION

Radiography techniques including x-ray and gamma can detect inclusions below the surface of

the weld. Traditional 2D shots make it difficult to realize the spacing between pores that are

actually in different planes within the weld. With computed tomography (CT), the weld can be

Figure 3 - Stress Strain Curve for Aluminum Tensile Samples with Porosity [5]

rotated to more clearly understand the position of the pores within the weld. Tungsten is easily

spotted and will illuminate bright white within the weld. These methods are more expensive

than other methods, but provide a permanent record of the weld inspection that be review

later as necessary. Ultrasonic testing will pick up slag and tungsten, but is not useful in finding

pores. Visual inspection, liquid penetrant, and eddy current can detect surface discontinuities.

These methods are inexpensive compared to radiography do not typically provide a permanent

record that can be re-checked later. Where possible a proof and leak test of the weld joint can

detect inclusions that extend through the weld from the inner to the outer surface [1].

3.2 CRACKS

3.2.1 TYPES AND CAUSES

Cracks describes a broad spectrum of indications that “results from localized stress that at some

point exceeds the ultimate strength of the material.” The residual stresses, caused by the weld

joining process, due to shrinkage, create these cracks. Cold cracks refers to cracks that occur

below 400F (usually at room temperature) and hot cracks refers to cracks that occur above

400F [2]. “With narrow, deep grooves a crystallisation takes place (on) all sides of the bead,

entrapping the remaining melt in the bead centre. With the occurrence of shrinking stresses,

hot cracks may develop” [8]. The chemical composition of the metals can lead to an increased

cold crack susceptibility [8]. Some alloys contain sulfur or phosphorus that cause solidification

cracks and some alloys have a high solidification temperature that contributes to cracking [6].

The weld joint design and weld method are also contributors to weld cracks [6]. Hydrogen

within the weld filler and humidity around the weld joint increase the risk of cold weld cracks;

the longer the materials are stored, the higher the potential water content of the electrode

coatings [8]. “Cracking is more likely to occur if the metal is either hard or brittle,” while a

ductile material may better withstand the stress concentrations, because it can yield locally [1].

There are two main categories of cracks described in the AWS weld inspection handbook (all

cracks noted in Figure 4):

The first category is weld metal cracking with three types of cracks. Transverse cracks occur

perpendicular to the weld axis and more likely to occur when the weldment is highly restrained.

Longitudinal cracks form most often at the center of the weld. These may initiate during the

weld process and propagate out to the surface as the weld cools. Crater cracks can develop at

the start and stop locations of the weld bead. They are typically star-shaped and may be

initiation points for longitudinal cracks [1].

The second category is base metal cracking with two types of cracks. Transverse cracks occur

perpendicular to the weld axis, occurring more regularly with high strength steels. Longitudinal

cracks run parallel to the weld are more commonly found than transverse cracks. “For fillet

welds, longitudinal base metal cracks may be divided into two types: (a) toe cracks, which

proceed from the toe of the fillet weld into the base metal, (b) root cracks, which proceed from

the root of the fillet weld and progress into the base metal” [1]. The root cracks can sometimes

be found on the opposite side from where welding occurred. Base metal cracking occurs when

the weld and base metal cool quickly due to temperature of parts, processes, and environment.

Arc strike cracks are a little different. They are formed by an unintentional arc outside of the

weld area, usually an error by the weld operator. They occur as the localized area is rapidly

heated, melted, and cooled, causing localized embrittlement leading to a crack [1].

Figure 4 - Types of Weld Cracks [8]

3.2.2 EFFECTS

These cracks create stress concentrations in the weld and in the base metal. The significance of

those cracks certainly depends on their type, size, and location. In addition to typical fatigue

crack propagation issues, the weaker heat affected zone properties may accelerate the growth

compared to a crack in the base metal outside the heat affected zone [1]. Stress corrosion

cracking (SCC) may also accelerate from weld cracks on high alloy metals at the weld toe [10].

Microcracks that were 5% of the cross section of a 2 3/8” wide by 1/2” thick fatigue test sample

reduced the fatigue strength in 10 million cycle by about 60% [2]. If an improper weld filler is

chosen relative to the base metal, the mismatch of the properties can drive cracks more rapidly

in fatigue [7]. “Solidification cracking susceptibility is exacerbated if there is mid-section bulging

in the weld cross-section profile due to locally increased strain” [6]. Rework of cracks, like

inclusions, can be a time consuming, expensive process. For particularly large or deep cracks, no

amount of rework may be able to correct them.

Figure 5 - Weld Cracks Create Stress Concentrations [1]

3.2.3 PREVENTION

The risk of formation of weld metal cracks can be reduced by adjusting the electrode position

and movement or electrical conditions, decreasing the electrode travel speed, preheating the

parts to reduce thermal stresses, sequencing the welds to balance the unintended shrinkage

loads, and avoiding conditions that allow rapid cooling [1]. When the welder must compensate

for a large weld groove, widening the weld to fuse the metal, the likelihood of cracks in the

weld. “Proper part fit-up and good joint design are both key in preventing” cracks. For the weld

joint, “make the depth (ratio) 5:1 to 2:1 the size of the width” [7]. “Post-weld heat treatment

should be regarded as essential to obtain maximum SCC resistance from welded joints in high

strength steels” [10].

For base metal cracking, “in the case of low carbon, medium carbon, and low alloy steels,

hardness and the ability to deform without rupture” depend on the alloy composition and their

rate of cooling after the weld. Generally, the temperature inputs during weld can be controlled

better with automated versus manual weld processes. The base metal can be preheated and/or

the local temperature of the environment surrounding the weld can be raised to decrease the

cooling rate [1]. A thick base material tends to increase the rate of cooling and should be

avoided if possible [1]. The addition of copper or other heat sinks that increase the rate of

cooling can lead to cracks. High alloy steels are more susceptible to base metal weld cracks due

to hardening and/or embrittlement during cooling. Hardenable steels have “variations in the

microstructure of the heat affected zone (cause differences in mechanical properties), which

can occur with variations in the cooling rate” [1]. For hardenable steels, the AWS weld

inspection handbook, recommends preheating the base metal, controlling the heat input during

weld, and ensuring the correct electrode and weld materials are used [1]. Filler metal and base

metal should have the same or similar compositions; differences increase the risk of cracks in at

the edge of welds [1]. Hobart Brothers recommends filler wire with a low hydrogen content

where possible. Filler metals that “contain high levels of hydrogen scavengers, including

fluoride, sodium and calcium that can combine with hydrogen to remove it from a cooling

weld” can reduce the risk or cracks [9].

3.2.4 DETECTION

Visual inspection, liquid penetrant, magnetic particle, ultrasonic, and eddy current can detect

surface cracks easily; however, the depth/severity of the crack cannot be assessed clearly.

These methods do not typically provide a permanent record that can be re-checked later.

Where possible a proof and leak test of the weld joint can detect cracks that extend through

the weld from the inner to the outer surface. Radiography techniques including x-ray and

gamma can detect subsurface cracks in the weld. Traditional 2D shots make it difficult to realize

the full dimensions of the crack within the weld. With computed tomography (CT), the weld can

be rotated to more clearly understand the dimensions of the crack within the weld [1].

3.3 INCOMPLETE FUSION/PENETRATION

3.3.1 TYPES AND CAUSES

Incomplete fusion describes “the failure to fuse together adjacent layers of weld metal or weld

metal and base metal” occurring “at any point in the welding groove” [1]. The part may not

locally fuse because the temperature of the weld process was insufficient to cause the surface

of base metal to melt. This lack of fusion may be caused by a layer of oxides or dirt not removed

during cleaning, that does not adequately dissolve when heated during the weld [1]. This

phenomena is often confused with incomplete penetration. Compare the incomplete fusion

defect examples shown in Figure 6 to the incomplete pentration examples in Figure 7.

Figure 6 - Lack of Fusion Defect Examples [8]

Incomplete penetration “describes the failure of the filler metal and base metal, or the base

metal alone if no filler metal is used, to completely fill the root of the weld” [1]. There are two

main causes: failure of the weld to melt to the full depth desired of the weld, insufficient filler

metal to reach the root of the weld joint [1]. However, like lack of fusion defects, a layer of

oxides or dirt may prevent full penetration of the weld, though this is less common [1].

3.3.2 EFFECTS

Incomplete fusions of the whole length joint or most of the joint means that the part is not

fused together and may be obvious if the part is handled after weld. The part may behave as if

it were never welded at all. For localized lack of fusion, the strength is reduced because the

metal is not fully fused as intended for the design [1]. Service failures due to lack of fusion are

Figure 7 - Lack of Penetration Examples [1]

not common; in such cases, the incomplete fusion more than 10-30% of the effective wall

thickness [2].

Incomplete penetration creates detrimental stress concentrations as the parts are not welded

at the root of the weld. These are affected by tension and bending loads. Shrinkage and

distortion can lead to cracks in areas of the weld bead with inadequate penetration [1]. The

cracks add to the stress concentration that already exists from the lack of penetration on the

back side of the joint. Over time these can cause fatigue failures [1]. “Mechanical fatigue tests

on pipe butt welds with lack of penetration on the inside surface” reduce fatigue strength by

more than 50% [2].

3.3.3 PREVENTION

Melt rate and weld travel speed must be properly balanced or the risk of inadequate fusion

increases. For example, if the melt rate is slow and the travel speed is fast, then incomplete

fusion will occur due to low performance, but if the melt rate is fast and travel speed is slow,

then incomplete fusion will occur due to poor pre-flow. The orientation of the weld head is

crucial to prevent lack of fusion. Lack of fusion can be reduced with a 45 degree angle of the

weld head relative to the parts for fillets [8].

The melt rate versus the weld speed also plays the same role in incomplete penetration defect.

The two must be balanced properly to achieve melt through the entire weld depth desired and

to allow the filler metal to adequately fill the root of the weld joint. The orientation of the weld

head is also critical to prevent lack of penetration. Incomplete penetration can be reduced with

a 90 degree angle of the weld head relative to the parts for butt joints [8].

3.3.4 DETECTION

Sectioned weld samples can verify the weld parameters before parts are weld to ensure there is

no lack of fusion or penetration. Visual inspection, liquid penetrant, ultrasonic, and eddy

current can detect lack of fusion, like they do for surface cracks. Radiography techniques

including x-ray and gamma can detect lack of penetration in the weld, but are do not work well

to find lack of fusion [1].

4 CONCLUSIONS

The causes of inclusions, cracks, and incomplete fusion/penetration can be attributed to

process conditions (like cleanliness, environment, and materials chosen for weld), improper

welder inputs or techniques, or poor joint design.

Though weld defects occur within all types of materials and all types of weld processes, each

has trade-offs that must be recognized. Manual GTAW of aluminum welds will introduce many

small pores to the weld and EB of aluminum welds will reduce the total number of pores, but

they are likely to be larger in size. Nevertheless, defects affect the material properties similarly.

One of the most common effects is the introduction of stress concentrations that reduce

strength. These risers play a significant role in fatigue failures of welded joints. Porosity does

not significantly impact the yield strength of the material. The weld cannot be made perfect,

but good cleaning techniques, correct weld parameters, and proper joint design, can

appreciably reduce the risks of weld defects and their negative effects. The methods for

detection of these weld defects are fairly similar, though some work better for finding certain

defects. This knowledge can be adapted to select only the inspection necessary for a design.

Note that the companion PowerPoint presentation contains additional discussion and images

related to porosity types, causes, effects, prevention, and inspection.

5 REFERENCES

[1] Welding inspection (2nd ed., pp. 44, 47-48, 50-53, 56, 113-121). (1980). Miami, Florida:

American Welding Society.

[2] Thielsch, H. (1967). The Sense and Nonsense of Weld Defects (1st ed., pp. 17-18, 21, 23, 25,

37, 46). Lake Zurich, Illinois: Monticello Books.

[3] Walsh, W., Leis, B., & Yung, J. (1989). Study to Determine the Influence of Weld Porosity on

the Integrity of Marine Structures. SSC-334. Retrieved April 24, 2014, from

http://www.shipstructure.org/pdf/334.pdf

[4] Rudy, J., & Rupert, E. (1970, July). Effects of Porosity on Mechanical Properties of Aluminium

Welds. Welding Journal, 322-S. Retrieved April 24, 2015, from

https://app.aws.org/wj/supplement/WJ_1970_07_s322.pdf

[5] Shore, R. & McCaulay R. (1970, June). Effects of Porosity on High Strength Aluminium 7039.

Welding Research Supplement, 311-s. Retrieved April 24, 2015, from

https://app.aws.org/wj/supplement/WJ_1970_07_s311.pdf

[6] FAQ: What are the typical defects in laser welds? (n.d.). Retrieved April 24, 2015, from

http://www.twi-global.com/technical-knowledge/faqs/structural-integrity-faqs/faq-what-are-

the-typical-defects-in-laser-welds/

[7] 6 Key Ways to Prevent Weld Failures. (2012, March 7). Retrieved April 24, 2015, from

http://www.hobartbrothers.com/news/111/523/6-Key-Ways-to-Prevent-Weld-Failures.html

[8] Dilthey, U. (2005, January 1). Welding Technology 2 - Welding Metallurgy. Retrieved April

24, 2015, from http://www4.hcmut.edu.vn/~dantn/course/Pdf/WeldingTechnology2-

English.pdf

[9] Understanding Weld Cracking, Its Causes, Consequences and Remedies. (2009, September

9). Retrieved April 24, 2015, from

http://www.hobartbrothers.com/news/48/523/Understanding-Weld-Cracking-Its-Causes-

Consequences-and-Remedies.html

[10] Gooch, T. (1974, July). Stress Corrosion Cracking of Welded Joints in High Strength Steels.

Welding Research Supplement, 287-s. Retrieved April 24, 2015, from

https://app.aws.org/wj/supplement/WJ_1974_07_s287.pdf