Can someone please explain?
This tool is called a McLeod rake, a tool commonly used in fighting forest fires or wild fires. It is used to clear vegetation in order to make firebreaks, and also to cut off stray branches. The metallic head part of the rake is often made out of steel with low-to-moderate carbon content of around 0.5 wt% carbon, making it a reasonably ductile material.
You and your friend decided to take a break to get some water. However, as you returned with cold drinking water, you were alarmed to find a sudden change in wind direction has brought some rogue embers right in front of you. Your friend rushes to grab his McLeod rake that he had left on the ground, but as he picked it up, he found a large burning ball of grass attached to the “teeth” of the rake. Almost by reflex, he dumps his drinking water onto the metal head to put out the fire. The water sizzled as it touched the hot metal and steam rose into the foggy air.
As your friend p
lowed through the dry grass, he accidentally hit the head of the rake on a large rock. To his dismay, several teeth of the rake broke off.
You knew why your friend’s normally ductile rake suddenly fractured in a brittle fashion. What do you think happened?
Based on the information provided, it appears that the sudden fracture of the rake's teeth can be attributed to a phenomenon called embrittlement. Embrittlement refers to a reduction in the material's ability to deform and absorb energy, resulting in a brittle fracture when subjected to stress.
In this case, it is likely that the combination of the hot metal head and the sudden cooling effect of the cold water caused thermal shock. Thermal shock occurs when there is a rapid and uneven temperature change in a material, causing it to expand or contract unevenly. This rapid temperature change can induce internal stresses within the material, making it more susceptible to fracture.
When your friend hit the head of the rake against a large rock, the combination of existing internal stresses from the thermal shock and the external force from the impact resulted in a brittle fracture of the teeth.
To further explain this concept, the carbon content mentioned earlier plays a significant role. Metals with higher carbon content tend to be more susceptible to embrittlement, as the carbon atoms can accumulate at grain boundaries, forming carbides. These carbides act as stress concentrators, making the material more prone to fracture.
In summary, the sudden fracture of the rake's teeth can be attributed to a combination of thermal shock caused by the hot metal head and the cold water, as well as the presence of carbides due to the carbon content of the steel.