The WelderDestiny Compass

Sensors in Welding - Issue #008

Wednesday, February 22, 2017 / Perth Australia / By Niekie Jooste

In this edition of "The WelderDestiny Compass":

• Managing Risk
• Uncertainty in Engineering
• Welding is a High Uncertainty Operation
• Reducing Welding Uncertainty with Sensors 

Managing Risk

Life is a risky affair. A great deal of time and resources are spent in trying to manage the risks to reduce them to tolerably low levels. 

In the engineering field we sometimes fail, and then we can end up with tragic results like bridges failing, buildings collapsing, aircraft crashing, ships sinking, off-shore oil rigs burning or pressure vessels or pipelines exploding. 

A great deal of the efforts in risk reduction within engineering is associated with reducing the variability associated with the manufacturing process of products and components. Especially welding is prone to many variations and variables that result in increased risk of engineering structures. 

In today’s The WelderDestiny Compass, we will start exploring how future technologies could assist in greatly reducing this risk. 

If you would like to add your ideas to this week’s discussion, then please send me an e-mail with your ideas, (Send your e-mails to: compass@welderdestiny.com) or complete the comment form on the page below.

Now let's get stuck into this week’s topics...

Uncertainty in Engineering

In engineering, there are many uncertainties. These uncertainties need to be compensated for during the design phase of a product, and some need to be controlled within limits during manufacture. Typical uncertainties for a welded structure are: 

• Variations in the strength of the materials. 
• Uncertainty in the loading conditions. 
• Uncertainty in dimensional tolerances. 
• Uncertainty in fabrication results. (e.g. weld quality and weld properties.) 

To compensate for all these uncertainties and variations during design, design factors (also called “safety factors”) are introduced. By introducing these design factors, the conservatism of the design is increased. It is not untypical for a design to have a cumulative design factor of 3 or 4. This means that if the designer knew all the relevant factors 100%, the structure or component could be one third of the strength and cost than the component with a design factor of 3. 

Under circumstances where the high design factors result in an uneconomic product, the design factors can be reduced by performing higher levels of engineering to confirm design loadings, and testing during fabrication to assure a tighter tolerance on material properties and fabrication quality.

Often the additional engineering requirements and testing can also be extremely expensive and result in great increases in project schedules. 

Communicating the extreme levels of quality assurance and control activities can also introduce risk into the whole manufacturing process. 

Is it possible that there are technologies out there that could result in reduced risk and lowered cost, resulting in significant efficiency improvements for welding?

Welding is a High Uncertainty Operation

Welding is defined as a “special process” within ISO 9000. What does this mean? It means that it is not possible to fully confirm the results of a weld without rendering the weldment unusable. In short, the only way to fully confirm the properties of a weld is to subject it to destructive testing. 

To overcome this uncertainty, welding is subjected to a qualification process, and the Welders are also subjected to a qualification process. The welding procedure qualification is qualified to be carried out within a narrow range of essential variables. The idea being that as long as the weld is performed within the allowable range of essential variables, then the result should be acceptable. 

A big issue is however that much of what we would really like to know about the weld, cannot be monitored practically during welding. Typically, we would like to know what the maximum temperatures are that the base metal reaches at different distances away from the weld interface. We would also like to know what the cooling rates are. All of this is required to estimate what the weld microstructure is going to look like, and even what the corrosion resistance will be for some materials. To date, these factors could not be measured directly, so we have used “proxy” measurements to estimate these values. 

A typical proxy measurement for deciding what the weld microstructure will be, is the welding heat input. It is a proxy measurement for the maximum temperatures and the cooling rates, when taken in conjunction with the material thickness, thermal conductivity and the pre-heats and inter-pass temperatures. 

Surely it would just be better to measure the actual cooling rates? It will significantly reduce the uncertainty with all these proxy measurements. 

Another big issue is the design allowances that are needed for welding distortion and welding residual stresses. If we could dynamically measure the actual welding distortion and the welding residual stresses, then we could again reduce the uncertainties that need to be addressed with high design factors. 

It starts being clear that we could significantly improve component efficiency while reducing risk associated with failure by knowing more about the results of the welding operation. 

We will spend a lot of time looking at technologies that could aid us with this outcome, which brings us to… 

Reducing Welding Uncertainty with Sensors

Modern welding equipment is being designed in a different way than the past. As an example, in the past the core of a welding power source would be designed, and then any sensors that were needed were added, almost as an afterthought. (e.g. Volt and Amp meter) Modern power sources are designed around the sensors that will be needed to make the modern power source viable and competitive within a connected and computerised world. This trend will only increase in the future. Eventually the sensors and what they allow the welding power source to achieve, will be the real selling point of future welding power sources. 

Not only will the welding power sources have many embedded sensors. As mentioned in previous editions of The WelderDestiny Compass, there is a lot of potential in sensors associated with the visual analysis of the weld in real time, that would typically be embedded in the welding helmet. 

In future editions of The WelderDestiny Compass, we will look at several sensors and technologies that could be used to achieve our aim of increasing efficiency while reducing risk. 

Today we look at field material analysis. Currently the two main technologies for material analysis in the field are X-Ray fluorescence (XRF) and optical emission spectroscopy. (OES) Of the two, OES is superior in terms of safety and analytical ability. This is because OES does not require the use of dangerous X-Rays, and can analyse for almost all elements that we would be interested in. XRF cannot detect the lighter elements such as Carbon, Sulphur and Phosphorous, which are very important for predicting the outcomes of the welding operation. 

OES works by using an arc to vaporize a very thin layer of the surface of the material, and then analysing the optical emission given off by the high energy vaporized atoms. It starts looking like it is not that much of a stretch of the imagination that this equipment could be incorporated into a detector that could be made small enough to use on a “real-time” basis during certain welding operations such as Gas Tungsten Arc Welding. (GTAW) 

This makes sense, because we already have the equipment to create an arc that will vaporize the material, and we already have the necessary shielding gasses (Argon) to shield the arc from interference from atmospheric effects. 

Is the ability to perform real time chemical analysis of the weld pool composition just around the corner? I believe so!

Yours in welding

Niekie Jooste

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