Martensite and Wire Breaks

Martensite is a hard, non-ductile phase of steel formed when the outside wire surface is heated above its critical temperature, followed by rapid cooling by the adjacent "cold" metal within the wire and the rope structure. As the affected area continues to bend, it will crack very easily, quickly spreading through the wire and eventually leading to complete wire breaks.

The only evidence that enough heat has been generated on the rope's crown to produce martensite is the presence of a martensitic film at the surface of the rope's outer wires. This area, however, is not visible to the naked eye so martensite is very difficult to detect until areas of broken wires appear.

 

Martensite may develop under field conditions such as the following examples:

• Over spinning sheaves in fairleads of draglines. As the rope reverses direction during the digging cycle, it grinds against the vertical fairlead sheaves that are still spinning in the opposite direction, producing friction and heat.
• Rope contacting a standing object. If the rope continually slaps against a solid, stationary surface, the result is extra friction, pressure and undue heat on the outer rope wires.
• Undersized sheaves. These may cause martensite as the rope is forced under pressure to conform to the sheave size. Such small movement under high loads can result in martensite.
• Rock damage. If the drag ropes are continually pulled through the roll at the front of the dragline, they will rip through the rock and dirt and may create martensite.

 

Once martensite has formed, the damage is done. You can't repair damaged rope, but you can take steps to help prevent it. The first step is to correctly identify the problem and find out exactly where the martensite is on your rope. Then, when you install the next rope, you can take steps to keep it from occurring again, including:

• Prevent over spinning sheaves. Apply a heavy, tacky lubricant in the throat of the vertical fairleads to help the rope start and stop the sheaves. This also keeps the sheave from grinding against the rope. Never start the digging cycle until the vertical fairleads stop spinning.
• Provide rope protection. Consider covering the area the rope contacts with a wood or plastic material to prevent direct rope contact with hard, stationary surfaces during operation.
• Change undersized sheaves. Because grooves that are too small can cause extreme friction and heat on your rope, make sure your sheave and rope diameter sizes are properly matched.
• On draglines, push the roll down. Never allow drag ropes to pull through the roll at the front of the dragline where they can rip through rock and possibly suffer frictional heating and mechanical damage. You can keep this from happening by clearing off the rocks before you dig, or by pushing down the roll and keeping the ropes out of the rocks during operation.
• Improve digging technique. Strive for smooth operation at all times without the sudden tugs and yanks that can bring on possible martensite damage.