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My galvanized rebar cracked soon after galvanizing. What happened?

The first thing that pops into a galvanizer’s customer’s  mind when their galvanized rebar cracks are the steel suffered from hydrogen embrittlement during the galvanizing process. This diagnosis is incorrect. If galvanized rebar cracks soon after galvanizing, the culprit is strain-age embrittlement, not hydrogen embrittlement or liquid-metal embrittlement. Hydrogen embrittlement is only a  concern when the steel has a tensile strength above 150 ksi,  and would only be seen in the field when the steel is under load. Liquid-metal embrittlement isn’t a concern unless the steel has already shown signs of cracking before it enters the galvanizing kettle. The indication liquid metal-assisted cracking has happened is the presence of zinc metal on the crack surface. This article will discuss strain-age embrittlement on reinforcing bar (rebar), and best practices for preventing it from happening.

What is Strain-Age Embrittlement?

Strain-ageing is a process where steel becomes very brittle in areas of high stress when exposed to elevated temperatures. Strain-ageing happens when steel experiences increases in strength and hardness due to induced stresses and has a  corresponding decrease in ductility and impact resistance. As the steel becomes harder, it also becomes more brittle. Strain-aging happens slowly at room temperature and much more rapidly at higher temperatures, such as that used during the galvanizing process. When the steel has incurred enough stress due to strain-aging, it can crack. Cracking due to strain-aging is called strain-age embrittlement.

Causes of Strain-Age Embrittlement

Strain-ageing is made up of two components. First, stresses are induced into the steel by cold working. The most common type of cold working done on rebar is bending. The second component is exposure of the highly stressed steel to a high heat source, such as in the galvanizing kettle.  Besides cold working, another factor that contributes to stain-aging is steel quality. Rebar is more susceptible to strain-aging because it is commonly made from low quality steel.  These types of steel are more susceptible to strain-aging because the steel has many impurities that congregate at the highly stressed points in the steel. This makes strain-ageing  more likely at these stress points.

How Stresses Are Induced into Rebar

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It is important to understand exactly how stresses are induced  into the steel. It seems simple enough to predict the highest  stress points in rebar after bending it, but it is not as simple as  it may appear. The diagram above right illustrates the area of highest stress  when bending rebar. When the steel is bent, it would seem  like the highest stress point would be at the apex of the bend.  This is not the case.  The area of highest stress is at the toe  of the bend, not the  apex. The toe of the  bend experiences the  highest stress because  as the rebar moves  to form the bend, the steel in the holding  fixture remains static.  This means one part of  the rebar moves while  the other part doesn’t  move. If the rebar  cracks, it will likely  occur in this area.

Prevent Strain-Age Embrittlement

There are several ways to prevent strain-age embrittlement of  galvanized rebar. The first method is to bend the rebar after  galvanizing; however, this can result in flaking and cracking of  the galvanized coating. Flaking and cracking of the galvanized  coating are not rejectable under ASTM A767/A767M. Those  areas can be repaired using zinc-rich paint. Heating the bend  areas with a torch to apply zinc solder is not recommended  because it can strain-age the steel due to the elevated  temperature of the torch. Another method to reduce the possibility of strainage embrittlement is to follow the bending diameter  recommendations in ASTM A767/A767M. By following  these recommendations, less stresses are induced into  the steel. If rebar is required to be bent tighter than these  recommendations, it can be stress relieved at 900 F to 1050 F  (480 C to 560 C) for one hour per inch of bar diameter.

Who Is Responsible for Preventing Strain-Age Embrittlement?

According to ASTM A143/A143M, the designer, fabricator,  and galvanizer are each responsible for preventing strainage embrittlement, although the responsibilities of each  are different. The designer of the product is responsible for  choosing proper steels to withstand normal galvanizing  operations without embrittling. The fabricator must employ  suitable fabrication procedures to prevent embrittlement. And  the galvanizer is responsible for using proper practices when pickling and galvanizing. Although strain-age embrittlement is a common concern,  by following the recommendations in this article and the  applicable specifications, the chances of the rebar embrittling  are greatly reduced. For more information please contact the  American Galvanizers Association at technical@galvanizeit.org  or 720-361-4485.