Effect of Sintering Atmosphere Control on the Surface Engineering of Catamold Steels Produced by MIM: A Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper is dealing with the influence of the sintering atmosphere onto the surface structure of different steels. The steels in the focus of the paper are representing the range offered by BASF for MIM and filament extrusion. Scientifically it makes more sense to focus on a technical topic than on the portfolio of a specific supplier. If you want to concentrate on sintering atmosphere for MIM, you should especially stress the specific characteristics of MIM and discuss the influence of the sintering atmosphere onto the surface with regard to these specific characteristics. Otherwise, it is only a general textbook on sintering atmospheres.

There are several possibilities for refocussing your paper. You could concentrate on e.g. MIM or Material Extrusion for metals and discuss the influence of the atmosphere on a specific metal or range of metals. You could concentrate also on a certain binder system, as in your case a POM-based system. Additionally, surface engineering means beside the material characteristics at the surface also the geometrical characteristics, especially for MIM and AM. In any case, you should refocus your paper, add additional literature and rework it.

Some detailed comments:

• Fused Deposition Molding is the trade name by Stratasys. According to the standards Material Extrusion is the right term. In your case it is Material Extrusion with filaments, which can be found in the literature also under FFF-Fused Filament Fabrication.
• line 42: use subscripts for '2' and 'n' in '(CH2O)n'
• lines 125-138: there are two sentences double
• lines 160, 170: 'Catamold'
• there are several times [see reference]: add the reference number

Author Response

Response Letter – Reviewer 1 
Dear Reviewer, 
We, the authors, sincerely appreciate your valuable contributions and have worked 
diligently to address and incorporate all your suggestions. Below are the points raised 
and the corresponding actions taken. 
1. The paper is dealing with the influence of the sintering atmosphere onto the surface 
structure of different steels. The steels in the focus of the paper are representing the 
range offered by BASF for MIM and filament extrusion. Scientifically it makes more 
sense to focus on a technical topic than on the portfolio of a specific supplier. If you 
want to concentrate on sintering atmosphere for MIM, you should especially stress the 
specific characteristics of MIM and discuss the influence of the sintering atmosphere 
onto the surface with regard to these specific characteristics. Otherwise, it is only a 
general textbook on sintering atmospheres. 
Response: Thank you very much for your suggestions. The review article has been 
revised with a focused scope on the Metal Injection Molding (MIM) process. Additional 
literature has been incorporated, and the entire text has been thoroughly reviewed. 
2. There are several possibilities for refocussing your paper. You could concentrate on 
e.g. MIM or Material Extrusion for metals and discuss the influence of the atmosphere 
on a specific metal or range of metals. You could concentrate also on a certain binder 
system, as in your case a POM-based system. Additionally, surface engineering means 
beside the material characteristics at the surface also the geometrical characteristics, 
especially for MIM and AM. In any case, you should refocus your paper, add additional 
literature and rework it. 
Response: Thank you very much for your suggestions. They helped us maintain a clear 
focus on the MIM process. 
3. Some detailed comments: 
Fused Deposition Molding is the trade name by Stratasys. According to the 
standards   Material Extrusion is the right term. In your case it is Material Extrusion with 
filaments, which can be found in the literature also under FFF-Fused Filament 
Fabrication. 
 line 42: use subscripts for '2' and 'n' in '(CH2O)n' 
 lines 125-138: there are two sentences double 
 lines 160, 170: 'Catamold' 
 there are several times [see reference]: add the reference number 
Response: Thank you very much for the suggestions. The text has been revised and 
highlighted in red as requested. 
We thank you for the high quality of your review and for the constructive 
recommendations provided. Sincerely, 
The Authors

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This review article addresses the effects of sintering atmosphere control on the surface engineering of Catamold steels produced via Metal Injection Molding (MIM) and Metal Additive Manufacturing with Fused Deposition Modeling (MAM–FDM). The manuscript examines the underlying thermodynamic mechanisms, the detrimental effects of inadequate atmospheres (decarburization and carbon enrichment), and identifies best practices for diverse steel grades. The review synthesizes scientific literature with the authors' three decades of technical experience in controlling atmospheres during thermal and thermochemical treatments. The analysis encompasses various atmosphere types (nitrogen, hydrogen, partially reducing atmospheres, low-pressure vacuum) and post-treatment processes (carbon recrystallization, carburization, nitriding).

Recommendations for Improvement (Maximum 15 Considerations)

1. Title: Typographical error and excessive length. The word "Engneering" contains a typographical error. Additionally, the title is excessively long and non-specific for a focused review on atmosphere control. Revised title: "Surface Engineering of Catamold Steels by Sintering Atmosphere Control in MIM and MAM-FDM Processes: A Review."
2. Inconsistent nomenclature for Catamold steels. Steel designations vary throughout the manuscript (e.g., "catamold 8620," "Catamold 8620," "Catamold 42CrMo4"). Standardize nomenclature according to ISO 683 specifications or the relevant standard for Catamold materials to ensure consistency and clarity.
3. Insufficient clarification of MAM–FDM terminology. While the acronym is defined in the abstract, reinitializing it in the introductory methodology section would benefit readers unfamiliar with this evolving technology and enhance accessibility.
4. Decarburization depth threshold inadequately justified. The critical decarburization depth threshold of 0.2 mm is cited repeatedly without quantitative references or comparative diagrams validating this value across the examined steel grades. Provide supporting data or cite specific experimental evidence.
5. Incomplete process temperature specifications. Although debinding temperatures (150–200 °C) are provided, exact sintering and pre-treatment temperatures for each steel class require specification, including acceptable tolerance ranges for process reproducibility.
6. Dew point control parameters lack quantitative guidance. While dew point control is identified as critical across multiple sections, specific quantitative recommendations (relative humidity ranges) for each atmospheric condition are absent, limiting practical implementation guidance.
7. Figure 2 insufficient documentation. The provided micrographs lack scale bars, magnification indicators, and quantitative analysis of defect depth and intensity. Complementary Energy Dispersive Spectroscopy (EDS) data would strengthen phase identification and validate observations.
8. Ambiguity regarding theoretical density achievement. The statement "theoretical density higher than 99%" requires clarification: does this refer to relative density by pycnometry or volumetric density? This ambiguity compromises interpretability.
9. Superficial treatment of diffusion kinetics. While the Ellingham diagram is presented, discussion of carbon diffusion kinetics (diffusion coefficients, characteristic times, activation energies) remains qualitative. Quantitative analysis of diffusion-limited penetration depths would strengthen technical rigor.
10. Inadequate systematic comparison between MIM and MAM–FDM. Despite both processes appearing in the title, critical differences in sintering parameters and process-specific atmospheric effects between MIM and MAM–FDM are not systematically compared, representing a missed opportunity for process differentiation.
11. Inconsistent and incomplete citation formatting. Multiple references exhibit formatting inconsistencies, including missing editorial details, incomplete DOI assignments, and irregular reference structures. Standardize all citations according to the journal's guidelines (e.g., References 8, 39, and others).
12. Absence of quantitative experimental data tables. The review relies predominantly on theoretical principles and qualitative observations. Comparative tables presenting microhardness data, measured decarburization depths, and corrosion resistance values for each recommended treatment would significantly enhance technical utility.
13. Transparency of methodology compromised by implicit data attribution. The incorporation of "30 years of personal experience" lacks specification regarding the proportion of quantitative data derived from the authors' background versus literature sources. Explicit delineation of primary versus secondary knowledge would strengthen methodological transparency.
14. Insufficient specificity in thermochemical treatment recommendations. Section 3.9 recommendations for thermochemical treatments (recarburization, carburization) lack specific thermal cycle diagrams and quantitatively defined carbon potential control parameters, limiting their direct applicability.
15. Conclusions lack critical synthesis and future perspectives. The conclusion section restates recommendations already presented without critically evaluating existing knowledge gaps, research limitations, or establishing a clear research roadmap for emerging MAM–FDM applications—a significant oversight for a forward-looking review.

Author Response

Response Letter – Reviewer 2 
Dear Reviewer, 
We, the authors, sincerely appreciate your valuable contributions and have worked 
diligently to address and incorporate all your suggestions. Below are the points raised 
and the corresponding actions taken. 
1. Title: Typographical error and excessive length. The word “Engenharia” 
contains a typographical error. In addition, the title is excessively long and not 
specific to a review focused on atmosphere control. Suggested revised title: 
“Surface Engineering of Catamolded Steels by Atmosphere Control in MIM and 
MAM-FDM Processes: A Review.” 
Response: Thank you very much for your suggestion. Indeed, adjusting the title 
is important. Based on this recommendation, we decided to focus the review 
article on Metal Injection Molding steels to improve the clarity and coherence of 
the text. Modifications are highlighted in red in the manuscript. 
2. Inconsistent nomenclature for Catamold steels. The designations of the steels 
vary throughout the manuscript (e.g., “Catamold 8620”, “Catamold 8620”, 
“Catamold 42CrMo4”). Please standardize the nomenclature according to ISO 
683 or another relevant standard for Catamold materials to ensure consistency and 
clarity. 
Response: Thank you for this suggestion. We revised the text to clarify that steels 
produced by the catalytic debinding route (Catamold) follow universal 
nomenclature. Examples of different steel grades are provided for each condition. 
Modifications appear in red in the manuscript. 
3. Insufficient clarification of MAM-FDM terminology. Although the acronym is 
defined in the abstract, reintroducing it in the introductory section of the 
methodology would benefit readers unfamiliar with this emerging technology and 
enhance accessibility. 
Response: Following your suggestion, we decided to focus the review article 
exclusively on Metal Injection Molding steels to strengthen the coherence of the 
discussion. Adjustments are highlighted in red in the manuscript. 
4. The decarburization depth threshold is not adequately justified. The 
commonly cited critical threshold of 0.2 mm is repeated without quantitative 
references or comparative diagrams validating this value across the analyzed 
steels. Please provide supporting data or cite specific experimental evidence. 
Response: Thank you for your important observation. We expanded the 
discussion on this point in the manuscript. The changes are highlighted in red. 
5. Incomplete specification of processing temperatures. Although debinding 
temperatures (150–200 °C) are provided, exact sintering and pre-treatment 
temperatures for each steel class must be specified, including acceptable tolerance 
ranges for process reproducibility. 
Response: Thank you for your valuable suggestion. We expanded and clarified 
the temperature specifications in the text. Modifications are highlighted in red. 
6. Control parameters for dew point lack quantitative guidance. While dew 
point control is repeatedly identified as critical, specific quantitative 
recommendations (relative humidity ranges) for each atmospheric condition are 
not provided, limiting practical applicability. 
Response: Thank you for this important recommendation. We expanded the 
discussion and added quantitative guidelines. Modifications appear in red. 
7. Figure 2 lacks adequate documentation. The micrographs do not include scale 
bars, magnification indicators, or quantitative analysis of defect depth and 
severity. Complementary Energy-Dispersive Spectroscopy (EDS) data would 
strengthen phase identification. 
Response: Thank you for the valuable feedback. Figure 2 has been revised as 
requested. Qualitative EDS microprobe analysis is indeed useful; however, carbon 
measurements may be unreliable due to technical limitations in biological-type 
samples. Nevertheless, decarburization and carbon enrichment effects are easily 
characterized using microstructure and microhardness analysis. 
8. Ambiguity regarding theoretical density measurement. The statement 
“density greater than 99% measured by pycnometry” requires clarification: does 
this refer to relative density or volumetric density? This ambiguity affects 
interpretation. 
Response: Thank you for this important observation. The revised text reads: “The 
material exhibited a density exceeding 99% of its theoretical value, as determined 
by pycnometric measurements”. 
9. Inadequate systematic comparison between MIM and MAM-FDM. Although 
both processes appear in the title, critical differences in sintering parameters and 
atmosphere effects between MIM and MAM-FDM are not systematically 
compared. 
Response: Thank you for the suggestion. The manuscript was revised to focus 
exclusively on the MIM process. 
10. Inconsistent and incomplete citation formatting. Several references display 
inconsistencies, missing editorial details, incomplete DOIs, or irregular structures. 
Please standardize all citations according to the journal’s style guidelines (e.g., 
References 8, 39, and 
Response: All references have been revised and standardized. 
others). 
11. Absence of tables with quantitative experimental data. The review relies 
predominantly on theoretical principles and qualitative observations. 
Comparative tables with microhardness data, measured decarburization depths, 
and corrosion resistance values for each recommended treatment would enhance 
technical relevance. 
Response: The methodology was adjusted, and the text was reformulated 
accordingly, with additions highlighted in red. Thank you for the suggestions. 
12. Insufficient specificity in thermochemical treatment recommendations. 
Section 3.9 recommendations for thermochemical treatments (recarburizing, 
carburizing) lack specific thermal cycle diagrams and quantitatively defined 
carbon potential control parameters. 
Response: Examples of thermal cycles for different steel grades were added. 
13. Conclusions lack critical synthesis and future perspectives. The conclusion 
section reiterates recommendations without critically assessing knowledge gaps, 
research limitations, or outlining a future research road mapa significant limitation 
for a forward looking review. 
Response: Thank you for your insightful suggestion. The conclusions have been 
revised accordingly. 
We thank you for the high quality of your review and for the constructive 
recommendations provided. Sincerely, 
The Authors

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The paper is now focussed and basically fine for publication. There are only very minor points to be corrected.

line 31: 'surface substrate'

line 104: are here really all references from 15 to 51 to be cited?Especially with higher than 99% theoretical denstiy? Please check and correct.

line 525: 'Practical'; check the formatting here.

References: adjust the formatting to unify it for all the references, especially italics.

Author Response

Cover Letter 

 

Dear Reviewer 1,

We sincerely appreciate your valuable contributions. All of your suggested improvements have been incorporated into the manuscript. We also took this opportunity to add several relevant points that further enhance the content of the scientific article. All modifications have been highlighted in green in the text.

1. Line 31: 'surface substrate.' Thank you very much for pointing this out. It has been corrected in the text.
2. Line 104: Are all references from 15 to 51 really necessary to be cited, especially regarding densities higher than 99% theoretical? Please check and correct. Thank you for this comment. It has been addressed in the text.
3. Line 525: 'Practical' Please check the formatting here. Thank you for the suggestion. The formatting has been corrected.
4. References: Adjust the formatting to unify all references, especially italics. Thank you for pointing this out. The references have been updated accordingly.

Sincerely,
The Authors

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The revised manuscript demonstrates moderate responsiveness to reviewer feedback, with complete compliance achieved for 7 of 14 reviewer requests (50%). However, critical gaps remain that necessitate substantial further revision.

 

1. Evasiveness Regarding MAM-FDM Integration (Requests 3 & 10):

The complete removal of MAM-FDM content represents a substantial non-compliance. Rather than providing the requested systematic comparison, authors eliminated the topic entirely, fundamentally altering the manuscript's scope without addressing the underlying scientific question.

2. Insufficient Microstructural Documentation (Request 7):

The claim that "technical limitations" prevent adding scale bars and EDS data is unacceptable for peer-reviewed publication. Quantitative microstructural characterization is essential for validating claims regarding decarburization and carbon enrichment.

3. Absence of Quantitative Experimental Data (Request 12):

The review remains predominantly theoretical. The promised "methodology adjustment" does not substitute for the requested comparative tables presenting microhardness values, decarburization depths, and corrosion resistance data.

Author Response

Dear Reviewer 2,

The authors sincerely appreciate your valuable contributions and would like to inform you that every effort has been made to address your suggestions. Although this work is a review article, it provides a relevant contribution by compiling and presenting best practices for the recovery of components whose surfaces may have been compromised by inadequate sintering atmospheres. Below, we present our responses to the comments and suggestions provided in your valuable review.

 

1. Handling of MAM-FDM Comparison (Requests 3 & 10): The complete removal of MAM-FDM content was considered a substantial non-conformity. A systematic, justified comparison is required. 

Answer:  We sincerely appreciate your comments. We would like to clarify that our intention was by no means to disregard any guidelines, but rather to focus on the more broadly applicable technology. The catalytic additive manufacturing process is currently limited to two materials, whose sintering is carried out in a reducing hydrogen atmosphere, making them less susceptible to the phenomena under study. These materials would only be affected if sintering were conducted in furnaces also used for the MIM process, which employs nitrogen and hydrogen atmospheres.Nevertheless, we have added a few additional points to the review and included a final note on this aspect at the end of the article. The scope has been systematically re-adjusted and the comparison re-introduced. We have added a dedicated paragraph to the Introduction (Section 1) that justifies the review's focus on MIM, contrasting it with MAM-FDM. This new content explains that MIM is the optimal solution for high-volume production and superior microstructural control, while MAM-FDM is better suited for prototyping. This quantitative difference is corroborated with the inclusion of Table 1 (Comparative Mechanical Properties of 17-4 PH MIM vs. MAM-FDM).

2. Absence of Quantitative Experimental Data (Request 12): The manuscript remains predominantly theoretical. Quantitative tables for microhardness profiles, decarburization depths, and corrosion data are required.

Answer: We sincerely appreciate your comments. New quantitative data and parameters have been inserted throughout the manuscript. In addition to Table 1, the manuscript now includes: a) Detailed quantitative thermal cycles for carbon recovery (e.g., 870 ^oC for 3h at 0.4% C-potential) in Section 3.9. b) Explicit discussion of critical impurity limits (O, C, N) and the use of the LECO method for quality assurance, per industrial standards (Section 3.1).

3. Insufficient Microstructural Documentation (Request 7): The justification for lacking quantitative data (scale bars, EDS) was deemed unacceptable. Microstructural characterization must be rigorously justified.

 Answer: We sincerely appreciate your comments. The methodology and figures have been updated and rigorously justified. The Materials and Methods (Section 2.2) was rewritten to: a) Explicitly state that Vickers Microhardness (HV) profiles are the preferred and more reliable quantitative method for determining carbon gradient depth over semi-quantitative EDS for light elements (C, O, N). b) Confirm that the precise quantification of C, O, and N is performed using Elemental Analysis (LECO Method). Furthermore, all microstructural figures have been verified to include scale bars and magnification indicators as requested.

Conclusion and References:

Conclusion and Future Roadmap: The Final Considerations (Section 4) has been completely rewritten to provide a critical synthesis, including a dedicated discussion on the current Knowledge Gaps and a structured Future Research Roadmap, addressing all aspects requested.

References: The reference list has been rigorously checked for formatting consistency (in English) and has been expanded to 61 items with the inclusion of new, high impact specialized literature (including a seminal AM review, a reactive MIM handbook, and recent titanium/copper MIM research) to support the added quantitative and comparative discussions. We believe that these comprehensive revisions fully satisfy all comments and concerns raised. We thank you once again for your professional guidance and look forward to hearing from you soon regarding the acceptance of our manuscript.

Sincerely,

The Authors

 

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The revised manuscript shows meaningful improvements and demonstrates the authors' effort to consolidate and focus the scope of the review. While the latest reviewer comments raise valid concerns regarding the exclusion of MAM-FDM comparison and the lack of detailed microstructural/quantitative data, these issues are partially offset by the technical completeness and clarity achieved in the current form.

 

The updated review provides a technically sound and well-organized synthesis of sintering atmosphere effects in MIM-produced Catamold steels, which remains a relevant and underexplored topic within the field of surface engineering and powder metallurgy.

Although the MAM-FDM integration was removed rather than elaborated, the revised manuscript now offers a more focused narrative that centers on MIM, improving depth and coherence.

While detailed microstructural and experimental data are still limited, the authors have refined their explanations, included literature-supported mechanisms, and expanded the theoretical framework sufficiently to support their conclusions.

The visual elements and section organization have been notably improved, enhancing accessibility for readers.

 

In conclusion, while certain aspects (e.g., direct microstructural quantification and expanded benchmarking) may be addressed in future follow-up studies, the current version of the manuscript is suitable for publication based on its scientific relevance, improved clarity, and its utility as a reference for process atmosphere optimization in MIM steel sintering.