Podcast Episode 12: Six-sigma and materials engineering

In this episode I discuss 6-sigma, statistical process control, and the role of materials engineering in developing capable manufacturing processes.

Materials are used to fabricate components and join components together. And, we want manufacturing processes capable of producing components and assemblies without defects or problems, and whose materials have the desired properties. So, developing and maintaining a six-sigma manufacturing process depends heavily on materials engineering.

The episode is at https://spotifyanchor-web.app.link/e/doXEJ1HOSzb

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A Tribute to Steel Metallurgy Knowledge

Our knowledge of steel has come a long way in the more than 2,000 years since it was first developed. We understand the effects of adding carbon and a wide variety of other alloying elements on steel properties. We understand the microstructures that form in steel, how they are influenced by steel composition and mechanical and thermal treatments, and their influence on steel properties. This knowledge has enabled extensive engineering of steel alloys, steel production processes, and steel component design and fabrication.

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Thousands of steel alloys

This knowledge has enabled the development of different types of steels to meet different applications. This includes carbon steels, low-alloy steels, tool steels, high-strength low-alloy (HSLA) steels, and stainless steels. Adding various combinations of alloying elements enable us to tailor steel strength, ductility, and other properties to meet a wide range of requirements for a wide range of applications. Each of the different types of steels has a certain set of alloy compositions and range of properties that make them suitable for different applications.

Ultra-high volume production

Our understanding of steel metallurgy has also enabled the development of many processes used to alter steel shape and microstructure to achieve specific properties and to produce different forms of steel. Steel mills use hot-rolling, cold-rolling, three different annealing processes, and normalizing to create microstructures that result in desired mechanical properties, easier machining, and/or better response to further heat treatment.

Currently, mills worldwide use these processes to produce about two billion tons of plate, sheet, and bar a year. Some sheet is used to produce welded tubing and pipe and some bar is used to produce wire and seamless tubing and pipe. Regardless of the form, steel metallurgy knowledge has enabled most mills to develop robust production processes capable of producing steel that consistently meets customer requirements.

Designing and making components

Then, companies that fabricate steel components from plate, sheet, bar, and tubing use through hardening heat treatments, such as quench and temper, to alter the steel microstructure to increase its strength and hardness. They also use case hardening heat treatments to alter the composition and/or microstructure of a component’s surface for increased strength and/or increased hardness. Because of their knowledge of steel metallurgy, most of the companies performing the heat treatments have developed processes capable of consistently meeting customer requirements.

Finally, companies that design steel components for use in their products can take advantage of steel metallurgy knowledge to optimize designs for performance, reliability, and cost. By understanding the effects of alloy composition, mill processing, and post-fabrication heat treatments on steel properties they can select the optimum alloy, mill condition, fabrication method, and post-fabrication heat treatment. Sometimes the considerations lead to innovations that give a competitive advantage in performance, reliability, and or cost.

Versatile and crucial

Steel is an incredibly versatile material that is used in a wide range of applications across numerous industries. It has come a long way since its early beginnings and plays a crucial role in shaping our modern world.

Ready to learn more?

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6-sigma and Materials Engineering

Every once in a while I notice a company’s website uses the term “six sigma” or “6σ” to convey good quality. When I see this I wonder if they really understand what six sigma is and what it applies to. This article is about that and, of course, how materials engineering fits in.

Sigma refers to standard deviation, a calculated value that describes the spread of data in a dataset. The larger the standard deviation, the larger the spread in the data. The figure shows two datasets of attribute measurements for two different sample populations. The y-axis shows the number of occurrences for each attribute value. Dataset 1 has a wider spread than dataset 2, so the standard deviation for dataset 1 is larger than the standard deviation for dataset 2.

Statistical process control

Sigma is used as part of statistical process control (SPC). SPC is a data-driven methodology used to monitor and control a process. The goal of SPC is to produce output that meets customer requirements, maintain output variations within acceptable limits, and minimize defects.

SPC can be used to control any process that produces a measurable output, such as the size of a component, the temperature of a machine, or the time to complete a task. It is used in a variety of industries, including manufacturing, healthcare, and service industries. The focus here is manufacturing.

With SPC, the attributes of the output of a process are measured, collected, and analyzed. Examples of attributes are part dimensions, metal hardness, or coating thickness. The data is used to identify and eliminate causes of process output variation and improve the quality of the process output. So, SPC is a quantitative approach for monitoring and controlling a production line to consistently produce output that meets specifications.

Six sigma

Standard deviation (sigma σ) is continuously calculated for the data collected. The goal, if you’re interested in having a capable process, is a small standard deviation with respect to the difference between the upper and lower specification limits of the attribute being measured. If ±6σ (12 standard deviations) is equal to the difference between the upper and lower specification limits, then only 3.4 out of 1,000,000 items produced will not meet specifications, i.e. 3.4 defects per million opportunities. Six-sigma!! This assumes the data average is centered between the upper and lower specification limits. There are other SPC calculations if the average is not centered.

SPC and materials engineering

How does materials science fit in with all this? Well, materials are used to fabricate components and join components together. Also, we want manufacturing processes to produce components and assemblies with materials that have the desired properties. So, developing and maintaining a six-sigma process and producing a “6σ” product depends on…

• Selecting materials with composition and properties that are compatible with the process, enabling good manufacturability.
• Characterize effects of incoming materials variations on process output.
• Controlling composition and properties variations of incoming materials. This includes writing good materials specifications and verifying that suppliers are capable of supplying materials and components that meet specifications.
• Selecting and controlling process conditions.
• Using failure analysis to help identify the root cause of problems.

Focus and discipline

SPC is a powerful tool for improving quality and reducing costs. As with anything else like this, if it was easy to do, everyone would be doing it. But it requires a commitment from management and employees to be successful. And it takes engineering focus and discipline to use the data to make process and design improvements.

If you’re interested in learning more, check out the video at the top of this post. It goes into more detail.

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A different perspective for seeing products

I have a different perspective than most people when it comes to how I see a product. This perspective influences how I approach component design, giving me an advantage for finding ways to reduce costs, reduce risks, and develop innovative solutions. I explain about this perspective in the short video below.

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Need help selecting materials for a component? CLICK HERE for help

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Improving Fatigue Resistance

Fatigue involves localized, permanent damage to metals exposed to cyclic stress. In a previous article I discussed the fatigue mechanism. This article covers factors that can be addressed to improve high-cycle fatigue life

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Factors that influence fatigue life

Several design, material, and fabrication factors influence component and joint fatigue life, including the following:

• Applied stress
• Metal strength
• Mechanical design features that are stress concentrators
• Non-metallic inclusions
• Fabrication defects
• Surface residual stress
• Surface roughness
• Metal fracture toughness

Applied stress
Fatigue life is inversely proportional to the stress on a component or joint. Sometimes, the easiest way to improve component and joint fatigue life is to reduce the load and/or increase component or joint cross-section.

Metal strength
Increasing an alloy’s strength increases the number of cycles before a crack forms. Strength can be increased by adding alloying elements, cold working, and/or heat treating. Steels can be made so strong that fatigue cracks do not form.

Keep in mind the trade-offs between strength and fracture toughness. For an alloy with a certain microstructure, as its strength increases its fracture toughness decreases and the crack length before final overload fracture decreases. See the discussion in this article on strength and toughness. So, while increasing strength can increase the number of cycles before crack formation, increasing strength too much can lead to fracture after a small crack has formed.

Need help figuring out the cause of a component failure or quality problem? CLICK HERE for help

Mechanical design features that are stress concentrators
Notches, holes, changes in x-section, and laser or scribed surface identification marks are examples of component features that are stress concentrators. Eliminating them or designing them to reduce the stress concentrating effect are ways to improve fatigue life.

Inclusions
Inclusions are nonmetallic and sometimes intermetallic particles in a metal that acts as stress concentrators and fatigue crack initiation sites. They are usually simple oxides, sulfides, nitrides, or their complexes in ferrous alloys and can include intermetallic phases in nonferrous alloys. Inclusions are the product of chemical reactions and contamination that occurs during metal melting and pouring.

Some alloys are produced using special processing and control over impurity levels to reduce the number of inclusions. Also, control over supply base is important – make sure metal comes from mills that have good control over their processes.

Fabrication defects
Fabrication defects include voids that form during metal casting and laps and seams that form during hot working processes. These defects are stress concentrators that can become crack initiation sites.

Surface residual stress
Residual stresses are locked-in elastic stresses within a metal, even though it is free of external forces (see this article on residual stress). Residual stresses can be tensile or compressive. In fact, tensile and compressive residual stresses co-exist within a component. Tensile residual stress at the surface of a component add to the tensile stress being applied, leading to reduced fatigue resistance. Compressive surface residual stress normally increases fatigue resistance because they subtract from the applied stress.

Cold working, steel through hardening (quench and temper), electroplating and other coatings, and welding are examples of processes that can result in tensile residual stresses at a component’s surface. Shot peening and other surface forming processes result in compressive surface residual stress and are used specifically for that purpose. Stress relief heat treating is used to reduce elastic stresses in components and weld joints

Need help designing a component? CLICK HERE for help

Surface roughness
Surface roughness acts as stress concentrators, reducing the number of cycles to initiate a fatigue crack compared to a smooth surface. The rougher the surface, the worse the fatigue resistance is for a metal. Different component fabrication methods result in different levels of surface roughness.

Fracture toughness
Fracture toughness is a measure of the ability of a material under load to withstand fracture when a crack is present. For two metal samples with the same applied load, the sample with the higher fracture toughness will be able to tolerate a larger crack before fracturing. Fracture toughness depends on the composition and microstructure of a metal.

Engineering for fatigue

Many approaches are available for designing and fabricating components and joints that have the reliability needed to withstand exposure to fatigue conditions. The trick is to identify the fatigue requirements for a component or joint and use the design and fabrication approaches that are easiest and least costly to implement.

Learn from failures

Finally, if you have components that are failing by fatigue, perform a failure analysis to determine the metallurgical and mechanical factors that are contributing to the failures to give you a better sense of the approaches to use to prevent the failures.

This article was originally published on the Accendo Reliability website https://accendoreliability.com/improving-fatigue-resistance/

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Podcast Episode 9: Learning and Professional Development

In this episode I discuss learning and professional development.

There’s so much to know to be an engineer and college courses cover the tip of the iceberg, even if you go to grad school. I learned this early in my career as I was faced with decisions and problems related to topics that I never encountered in school. I also learned that I had to shoulder the responsibility of learning the topics. Some of the learning came from colleagues, some came from taking short courses, and some came from reading textbooks and technical journals.

I this episode I discuss my learning experiences and philosophy about learning, the pitfalls of relying on learning just from experience, and resources for learning and professional development.

Here's a link to the episode. The episode is about 12 minutes long.

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Failure Analysis Case Study

Here's a video of an example of how failure analysis was used to improve the reliability of a component and the uptime of the equipment in which it was used. The information from the failure analysis was used to determine the root cause of the failure and the corrective actions to prevent future failures.

Interested in learning more about failure analysis of component failures and quality problems? Check out our failure analysis webinar and our failure analysis course.

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