Near Solidus Forming (NSF): Semi-Solid Steel Forming at High Solid Content to Obtain As-Forged Properties
Round 1
Reviewer 1 Report
The paper “Near Solidus Forming (NSF): Steel Semi-Solid Forming at High Solid Contents Obtaining as forged Properties” is very interesting and provides a lot of information about the near solidus forming process. However, the knowledge contained in the paper is very general and needs to be extended to include the exact parameters of the forming process and other experimental conclusions. The research team conducted a large volume of tests on many steels. Reducing the number of materials described and describing them in more detail would be appropriate. In its current form, the paper provides information on the possibility of forming the steels using the NSF method, but the processes in the materials during forming are not explained. Further mechanical testing is also necessary to assess the suitability of this forming method.
Since the parameters of the forming process are confidential, it is not possible to assess the success of this experiment. If it is not possible to indicate the forming temperatures of the individual materials, it would be useful to indicate the expected amount of liquid phase during the forming.
line 44 – 60 % of the solid fraction is not sufficiently large volume to compare forming of material in this state to forging. line 69 – “In this sense, the research group based on Mondragon Unibertsitatea (MU) has also been active in studying this field.” – a weird way of the formulation. line 70 – “However, the working temperatures used for the manufacturing of components are even lower, below the solidus temperature measured by DSC, which implies that there should not be any liquid. Nevertheless, the semi-solid material behaviour benefits are equally achieved, reducing the raw material and energy consumption whilst attaining as forged mechanical properties.” How is that possible when thixotropic behaviour occurs only in partially melted material? The press capacity is presented in the paper, but the forces needed for the forming of each material are not mentioned. Line 102 - using a lower frequency does not overheat the surface. The charge is heated homogeneously. Chapter 2.2. should be rewritten. Its introduction does not provide any important information; it could be erased. Parameters of forming for individual materials must be given in the table. It is not comfortable to look for all the parameters in the text. Also without knowing the forming temperature of individual materials, it is hard to assess the whole forming process and thus to adequately understand the results. Is it possible to obtain homogeneous mechanical properties across the whole part? Even in all directions? Did the research group tested mechanical properties in other directions than indicated in Fig. 5? Scale bar for Fig. 6a is missing Graph showing the entire thermal cycle would be useful The increase in ductility of 42CrMo4 and S48C should be explained in more detail. At the same time, it is insufficient to observe the bainitic structure by light microscopy. SEM should be used to determine the character of the bainitic structure. Line 309 - if Widmanstätten ferrite is present in the structure, considerable heterogeneity of mechanical properties can be expected in the individual regions. This must be verified. The different grain sizes in the final part will also greatly contribute to the heterogeneity of the mechanical properties. Scale bar for Fig. 10a is missing Line 351 – de deformation -> during the deformation?
Author Response
ARTICLE: Near-Solidus Forming (NSF): Semi-Solid Steel Forming at High Solid Content To Obtain As-Forged Properties
Comments from REVIEWER 1
1.- However, the knowledge contained in the paper is very general and needs to be extended to include the exact parameters of the forming process and other experimental conclusions
We have included in the introduction that the working range of the NSF process starts at 0.9 times the solidus temperature and goes up to the 5% liquid content. Furthermore, we have included a new table, Table 3, where exact temperatures at the end of the heating cycle are listed for each material. Other process parameters are also shown in that table.
The forming parameters are shown in Figure 3 and are the same for all the steel grades.
2.- The research team conducted a large volume of tests on many steels. Reducing the number of materials described and describing them in more detail would be appropriate. In its current form, the paper provides information on the possibility of forming the steels using the NSF method, but the processes in the materials during forming are not explained.
The main goal of the paper is to demonstrate that NSF is an alternative process for manufacturing complex components in a single step with as-forged mechanical properties. Thus, we have tried to demonstrate the applicability of the process to a wide range of steels. Studies to analyse deformation mechanisms inside the material during deformations are underway.
3.- Line 44 – 60 % of the solid fraction is not sufficiently large volume to compare forming of material in this state to forging.
Ok, we have removed it. In any case, solid fractions above 70% in compression show granular behaviour and crack initiation as referenced article 11 show, what means that it behaves more like a solid than a liquid.
4.- Line 69 – “In this sense, the research group based on Mondragon Unibertsitatea (MU) has also been active in studying this field.” – a weird way of the formulation.
Sentence erased.
5.- Line 70 – “However, the working temperatures used for the manufacturing of components are even lower, below the solidus temperature measured by DSC, which implies that there should not be any liquid. Nevertheless, the semi-solid material behaviour benefits are equally achieved, reducing the raw material and energy consumption whilst attaining as forged mechanical properties.” How is that possible when thixotropic behaviour occurs only in partially melted material?
When it is mentioned that below the solidus temperature the semi-solid benefits are achieved we refer to the possibility of filling the die with low forces, in a single step and obtaining near net-shaped components with as-forged properties and these are semi-solid processing advantages. We mentioned during the article semi-solid concept but not thixotropy since it is not demonstrated to occur with this high volume of solid. As is written at the beginning of the article the material behaviour is considered granular moving away from the thixotropic behaviour. When working at temperatures around solidus some liquid can be generated during deformation. At those conditions, there could be granular flow whose behaviour is similar to that from dense sand but it is not studied if it is thixotropic or not.
6.- The press capacity is presented in the paper, but the forces needed for the forming of each material are not mentioned.
The recorded forces are much more influenced by the geometry of the component than by the selected materials. Thus, peak forces mainly represent press frame deformation once the die is filled and not the material resistance to be deformed. Table 8 shows the peak loads with the 42CrMo4 and S48C. Loads measured for the 44MnSiVS6 and EN 13262-ER7 are also in the same range (see sections 3.3 and 3.4),) but as they are not used in hot forging they are not included in Table 8.
7.- Line 102 - using a lower frequency does not overheat the surface. The charge is heated homogeneously.
We have changed the line. It is a well-known effect and in our case taking into account the devices at our disposal we defined a heating cycle with several stages.
8.- Chapter 2.2. should be rewritten. Its introduction does not provide any important information; it could be erased.
We delete the first paragraph of the NSF process (2.2). We have changed substantially this section and we have sent the article to English editing and probe reading correction service.
9.- Parameters of forming for individual materials must be given in the table. It is not comfortable to look for all the parameters in the text. Also without knowing the forming temperature of individual materials, it is hard to assess the whole forming process and thus to adequately understand the results.
We have included a new table, Table 3, where exact temperatures at the end of the heating cycle are listed for each material. Other process parameters are also shown in that table. The forming stage is shown in Figure 3.
10.- Is it possible to obtain homogeneous mechanical properties across the whole part? Even in all directions? Did the research group tested mechanical properties in other directions than indicated in Fig. 5?
We do not test the properties in all directions. We have subjected the components to the same trials as the hot-forged ones because the objective was to make sure that as-forged mechanical properties could be achieved. In future works, we will explore this interesting line you mention.
11.- Scale bar for Fig. 6a is missing
Scale bar included.
12.- Graph showing the entire thermal cycle would be useful.
We can not show this cycle as it is confidential from the company.
13.- The increase in ductility of 42CrMo4 and S48C should be explained in more detail.
This is a point that we have to analyse in the near future using SEM and EBSD as you mention but it is an ongoing long term study.
14.- Line 309 - if Widmanstätten ferrite is present in the structure, considerable heterogeneity of mechanical properties can be expected in the individual regions. This must be verified. The different grain sizes in the final part will also greatly contribute to the heterogeneity of the mechanical properties.
You are wright but to get a homogeneous microstructure depends on the cooling rate and this, in turn, on the opening time of the tooling that will be improved.
15.- Scale bar for Fig. 10a is missing
Corrected.
16.- Line 351 – de deformation -> during the deformation?
Corrected.
Reviewer 2 Report
English should be edited, there are issues with clarity
Separate conclusions from discussion
Specify differences in mechanical properties between NSF and conventional processing for the same compositions (including in conclusions)
Author Response
ARTICLE: Near-Solidus Forming (NSF): Semi-Solid Steel Forming at High Solid Content To Obtain As-Forged Properties
Comments from REVIEWER 2
Separate conclusions from the discussion
Made.
Specify differences in mechanical properties between NSF and conventional processing for the same compositions (including in conclusions)
Differences in mechanical properties are between traditional hot forging and NSF are shown in Table 9 for the S48C steel. The 42CrMo4 alloy was compared with the OEM’s standards for hot-forged components as shown in Table 8.
The other two steel grades were not mechanically tested. The objective with the 44MnSiVS6 was simply to demonstrate that micro-alloyed steels are also suitable for NSF and the EN 13262 ER7 grade steel was used to confirm that as-cast steels could also be good candidates to be processed by NSF.
Reviewer 3 Report
General Remarks
This paper presents important data on industrial application The abstract must be integrated most technical and scientific results The heating billet is presented , but what is the heating temperature for each steel alloys ? Fig 2 : specify the steel alloy There is a difference between software and DSC è so what is the “god” temperature for these experiments ? Specify that chemical composition is in %wt The microstructures are not the same before mechanical characterisation and heat treatment are not specify. So discussion is difficult. For example, table 7 presents Re (HF) > Re(NDF) but without +/- Fig 7 : use the same scale or add two microstructures for the same scale NSF uses one step è authors must confirm that material flow doesn’t generate default … 21 references but only 3 refs after 2015 …
Author Response
ARTICLE: Near-Solidus Forming (NSF): Semi-Solid Steel Forming at High Solid Content To Obtain As-Forged Properties
Comments from REVIEWER 3
The abstract must be integrated most technical and scientific results
NSF process capabilities to reduce material consumption by 20%, reduce forming loads from 2100 t to 300 t are included.
The heating billet is presented, but what is the heating temperature for each steel alloys?
Table 3 is included, where temperatures for all the tested alloys recorded throughout the billet at the end of the induction heating are listed. This are the temperatures to achieve as-forged mechanical properties using 42CrMo4 and S48C steel grades.
Specify that chemical composition is in %wt.
Corrected.
The microstructures are not the same before mechanical characterisation and heat treatment are not specified.
In the case of the 42CrMo4 and S48C presented microstructures are from heat-treated components form hot forging and NSF. The microstructures displayed from the 44MnSiVS6 and the EN 13262-ER7 are obtained directly after NSF.
For example, table 7 presents Re (HF) > Re(NSF) but without +/- Fig 7
The specification values from the OEM have been introduced to see easily that NSF components are in the required range.
Use the same scale or add two microstructures for the same scale.
Changes implemented.
NSF uses one step è authors must confirm that material flow doesn’t generate default
We confirm that the material flow does not generate any default. The following tomographic analysis of the R spindle was presented in the article “Conformado semisólido (tixoconformado) de aceros para components de automoción”, Dyna, 2016, Vol. 91 nº 3, DOI:http://dx.doi.org/10.6036/7171.
See the figure in the attached file
There, it can be seen that the flow of the material during filling generates some folds that go up starting at the rounded edges of the base of the same. In any case, these have been analyzed and more than defects can be considered as surface irregularities since they do not enter the component. In the rest of the zones, the component is healthy.
Image quality was affected depending on the area examined. Thus, when the beam passes through areas with a large amount of mass, low-quality images are obtained, as can be seen in Fig. (c). On the other hand, the phenomenon of diffraction has been visible in various areas. One of them is shown in Fig. (b).
21 references but only 3 refs after 2015 …
Agree with the comment but unfortunately, nowadays semi-solid forming of steels is not a strong research issue. We hope this will change with the promising results showed in this article.
Author Response File: 
Author Response.pdf
Round 2
Reviewer 1 Report
The reviewer would like to thank the authors for clarifying the questions and completing the paper as it was requested. The authors made significant and beneficial changes.
Author Response
Dear reviewer,
English language and style checked and modified as you can see in the resubmitted article. Please, feel free to recommend any further changes. Regarding conclusions, please let us know which is the issue we should include to improve them.
