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The Five Top Causes of Spring Failures and How You Might Prevent Them By Luke Zubek, PE (Editor’s note: In this issue of Flashback we reprint a popular “how to” article from a former SMI staff member that originally appeared 10 years ago in the Fall 2010 issue of Springs.)
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xamining spring failures over the past years has inevitably led to me to recognize many trends in the causes, alloys affected, and commonalities encountered. I have prepared this article in an effort to summarize my observations on the top five causes and, more importantly, to address preventative actions. The causes and preventions reflect my own experience and observations and are in no particular order of frequency or importance. When submitting any defective material for failure analysis it is important to provide the consulting engineer with accurate fabricating details, service conditions and operating details. Failure to supply the correct background information can lead to non-conclusive or inaccurate conclusions.
Number One: Hydrogen Embrittlement (HE) HE occurs when three factors are present:
1. A tensile stress of sufficient magnitude either applied or residual. 2. A susceptible material is being used; the most susceptible spring materials are martensitic steels with high hardness (> 35 HRC), containing specific alloys and residual elements. The “poster child” of susceptibility is ASTM A401 or chrome silicon (CrSi). 3. Hydrogen content above a threshold level (depending on the above two factors only a few parts-permillion are necessary). The source of the hydrogen can be external to the spring like an acid wash. More commonly, the source of hydrogen is already present in the steel; the amount of residual hydrogen in non-degassed steel can be as high as 10 ppm. HE has also been referred to as delayed cracking since the embrittlement process takes time to occur.
Hydrogen atoms diffuse to the grain boundaries that are under tension. As time progresses, the pressure in these areas builds up, ultimately causing the steel to fracture intergranular. After the steel fractures, the detrimental hydrogen is locally liberated and ductility returns to the steel. This mechanism is directly reflected in the fracture features; the transformation from brittle to ductile features is one of the main fingerprints of hydrogen embrittlement. The most common causes of hydrogen embrittlement in springs are a delayed stress relief, skipped bake after plating, and hydrochloric acid cleaning. Knowing these causes can help prevent HE from occurring. Some suggestions to help prevent HE: • Stress relief immediately after coiling and use an in-line oven. • Document coiling times and stress relief times on CrSi grades. • Bake soon after electro-plating. • No hydrochloric acid cleaning on oil tempered grades. • Cautiously use high tensile CrSi grades and use music wire whenever possible. • Use a degassed steel that has intentionally low residual hydrogen (~1 ppm)
Number Two: Surface Quality Most surface quality issues that I have dealt with occur on stainless steel grades like 17-7PH and 302SS. These seams or folds are usually just within the required specification and reduce the operational life by facilitating fatigue cracks to initiate. Sometimes the surface takes on an appearance that is described as looking like a turtle shell or alligator skin. Fatigue typically initiates from one of the deeper folds or seams that are oriented between the grains. These areas are essentially the grain boundaries that are etched out during scale removal. It follows that as the grain size is reduced so is the depth of the seams,
SPRINGS / Fall 2020 / 39
