The hardness of higher value is brittle in nature. More grade in number means more alloying element are present. In performing tensile test of the above specimen, first start with high grade heat treated alloy material ( AISI 4340) which is 3 inch shaft test specimen. As, the load on it, more stress is applied on it. It will give one set of ultimate and breaking stress. Then go for the (AISI 4340) 6 inch shaft test specimen, it will show low ultimate and breaking stress, because stress decreases as area of the cross section increased.
After this, go for low grade alloy (AISI 4140) as followed above diameter specimen (3 inch followed by 6 inch) to determine the respective stress (ultimate and breaking) values.
To have high strength, the specimen has to undergone heat treatment process such as aeging or annealing for respective Re-crystallization tempertature. The material which has higher hardness indicated that it has high strength.