Question

The cutter bars of two combine harvesters from different manufactures develop different failure types with time. Cutter bar A loses the knife sharpening after 10 seasons, while cutter bar B does not lose the knife sharpening but some blades have to be replaced time to time due to local fracture of the edge. Through measuring the volume and the weight the density of both blades are around 7.8 – 7.9 Kg/dm³. The hardness of the blades from A is 2.2 GPa while the hardness of the blades from B is 8 GPa. Discuss two possible manufacturing differences that would lead to this situation. Use suitable references to support your discussion. Suggest two possible technologies to improve the life for bar A. Suggest a way to reduce the brittleness of bar B. Justify your suggestions and compare with real examples if possible.

[Hint] Use heat treatment graphs, composition-mechanical properties, material data, surface engineering…

Answer 1

Now let us study this entire case in a per case basis as per the table below:

Different cases to be studies under for the samples

Parameter	Case 1	Case 2	Case 3	Case 4
Sample	A	A	B	B
Density ()	7.8	7.9	7.8	7.9
Hardness (GPa)	2.2	2.2	8	8
Weakness	loses knife sharpening after 10 seasons	loses knife sharpening after 10 seasons	Time to time replacement of the blade.(Reason=local fracture at the edge)	Time to time replacement of the blade. (Reason=local fracture at the edge)

Here, from cases 3 and 4, there is clear evidence that the material that is employed in the making of blades from sample B is leading to a brittle behaviour. Indeed from material science books esp. Manufacturing Process by Hajra Choudary , we know that blades are made from soft iron. This is because hard iron is amicable for sharpening. Thus Sample B is a different material or a different combination of materials than sample A. The material in A is not having capacity to hold the sharpness all together. That means unlike normal materials like stainless steel, the material in sample A allows the mounding process . That says, the elastic behaviour of the compositions is very good. What leads to behaviour is steel is the low amount of carbon presence in the sample A. We need to cold work the knifes of sample A under the presence of hydrocarbons(aliphatic) gases in order to reduce the elongation properties. Nitrogen presence in sample A should be introduced during knife-making, whose general trend is to make the yield strength improve by implementing it under solution hardening, especially with austenite variety of steels.

For the knifes of Sample B to experience superior performance one has to change the materials that are used as additives during the manufacturing process.. Observe the following paper for further details:

https://inis.iaea.org/collection/NCLCollectionStore/_Public/37/088/37088593.pdf?r=1&r=1

Other reason for mounding are corrosion (which can be overcome by nitriding or galvanizing or electroplating), pitting (prevented by repeated alkaline treatment) and hydrogen embrittlement (seelink to overcome this case: https://www.brighthubengineering.com/machine-design/118961-fusion-of-inox-steel-and-embrittlement-issues/)

Now as far as the material behaviour on different manually forged knives are concerned, we need to observe this paper: https://aip.scitation.org/doi/pdf/10.1063/1.4985480

kg(dm) -3