Effect of Continuous Casting Practice on the Case and Core Properties of Carburized Alloy Steels

In the previous posting, the strain-controlled fatigue properties of the high-hardness case and low-hardness core of carburized SAE 4620 steel were compared.  It was shown that at short lives, the softer core exhibited better properties than the high-hardness case.  At long life, the fatigue properties of the high-hardness case were found to be superior. 

Again using SAE 4620 steel, AISI undertook a study to determine what if any changes in fatigue properties could be expected by changing continuous casting practice during steel manufacturing.  The steel described in the previous posting was continuously cast into 175mm by 175mm billets, and hot rolled to 50.8mm diameter bars.  For comparison, a second heat of SAE 4620 was evaluated that had been continuously cast to 279mm by 375mm blooms, and hot rolled to 47.6mm diameter bars.  For the billet cast steel the reduction in area during hot rolling was 93% and for the bloom cast steel it was 98%.  In both instances, the properties of the case were developed through simulation by diffusing carbon completely through fatigue specimen blanks. The properties of the core were simulated by subjecting specimens to the carburizing thermal cycle absent the presence of carbon in the atmosphere.
 
The mechanical properties and hardness values obtained for the case and core, both for the billet cast and bloom cast steel, are shown below:

     Casting    Location    Yield Str.   Tensile Str.    Red. in Area     HRC
     Practice                         MPa             MPa                 %                                                               

      Billet         Core            891.5           963.7              61.9                29.6

      Billet         Case         1169.1         1775.6                1.4                54

      Bloom       Core             687.5          997.8              58.2                29

      Bloom       Case          1316.6       2226.9                 4.0               59

In both cases, the case microstructure was 100% martensite, and a mixed bainite-martensite microstructure was developed in the core. 

Figure 1 (taken from the previous posting) shows the strain controlled fatigue properties for both the case and the core of the billet cast SAE 4620.  The strain-life curve for Iteration No. 53 shows the fatigue behavior of the core, and the strain-life curve for Iteration No. 54 shows the behavior for the high-hardness case.


Figure 1

 
Figure 2 shows the fatigue properties for both the case and core of the bloom cast SAE 4620.  The strain-life curve for Iteration No. 47 shows the fatigue properties for the core, and the strain-life curve for Iteration No. 48 shows the properties for the high-hardness case.
 


Figure 2

The two graphs show that both billet cast and bloom cast carburized SAE 4620 behave in a similar fashion.  At short life, the core exhibits superior fatigue properties, whereas at long life the properties of the high-hardness case exceed those of the core.  For each set of data a cross over occurs near 104 reversals.  The data also suggest that irrespective of the casting practice, the high-hardness case may be vulnerable to cracking as a result of periodic overloads.

Figure 3 compares the fatigue properties of the core for both bloom- and billet-cast SAE 4620, and Figure 4 gives the same comparison for the high-hardness case.  In Figure 3 the strain-life curve for Iteration No. 47 shows the fatigue properties for bloom cast SAE 4620, and the strain-life curve for Iteration No. 53 shows the properties for billet-cast SAE 4620.  In Figure 4, the bloom cast results are given by Iteration No. 48, and the billet cast results by Iteration No. 54.


Figure 3


Figure 4

It can be seen that the core properties for the two casting practices are nearly identical.  The case properties of bloom cast steel, however, do show a slight advantage over the billet cast steel in the very short life, high strain amplitude region.  Thus it appears that at all but the highest strain amplitudes, billet and bloom cast carburized SAE 4620 will perform equally.  At very high strain amplitudes, the carburized case of the bloom cast steel appears to be somewhat less vulnerable to cracking that may result from “spike” loading.

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