Carbon content in tamahagane steel

[Habaxi]

What is the minimum and the maximum carbon content in tamahagane steel? and what is the optimal carbon ratios for a sword made of tamahagane? Also how did ancient swordsmiths know and determine the other metals content in the steel to ensure their swords were functional? 

[AlexiG]

5 hours ago, Habaxi said:

What is the minimum and the maximum carbon content in tamahagane steel? and what is the optimal carbon ratios for a sword made of tamahagane? Also how did ancient swordsmiths know and determine the other metals content in the steel to ensure their swords were functional? 

Hi Hiadir,

 

I will offer an oversimplified answer based on few references. Carbon content varies in tamahagane. Part of the mastery of swordsmiths is to separate the pieces based on carbon content and select the appropriate pieces that meet their needs before they start forging. There are methods to both increase and decrease carbon content during forging to get the desired results.

 

I have seen good descriptions of the process in English from Yoshindo Yoshihara's books published with Leaon and Hiroko Kapp.  (The Craft of the Japanese sword)

If memory serves me right, he starts at about 1.2% carbon and wants to end at 0.7 % at the time of quenching. His brother though, supposedly preferred ~0.6% carbon. My guess is that these values refer to the outer, edge steel. I assume that varies from maker to maker and school to school.

 

How to the smiths "know" the carbon content without modern gadgets? Great question. I think how brittle the tamahagane is initially gives some indication for sorting. Beyond that I imagine it is about experience and how the steel responds to forging. I am speculating here

 

I hope this helps.

 

Alexi

[Franco]

Quote

 

Hagane (Steel) ･ ･ ･ Carbon content 0.03 to 1.7%. Can be stretched if you heat and hit it.

 

https://nbthk-ab2.org/sword-characteristics/

 

 

https://www.youtube.com/watch?v=gxwWf-MfZVk&t=300s

[Jacques]

The carbon content depends on the amount of material and the temperature. During the Heian, Kamakura, Nanbokucho and early Muromachi periods, blacksmiths produced their steel in modest-sized furnaces, obtening carbon content averaging no more than 0.45%. Later, the industrialization of steel production led to high carbon contents in Shinto and too high in Shinshinto, requiring more extensive folding to lower the carbon content, which in turn led to muji hada.
 

[ROKUJURO]

On 3/29/2025 at 11:04 PM, Habaxi said:

What is the minimum and the maximum carbon content in tamahagane steel? and what is the optimal carbon ratios for a sword made of Tamahagane? Also how did ancient swordsmiths know and determine the other metals content in the steel to ensure their swords were functional? 

Haidar,

TAMAHAGANE is produced in a range of different qualities. This is not intentional, but happens accidentally in the TATARA process. The basic material has low carbon content (around 0.3%), while some of the material is coming out of the TATARA as cast iron with more than 2% of carbon. The low-carbon TAMAHAGANE is mostly used for making SHINGANE, while cast iron is processed in OROSHIGANE furnaces to increase the carbon content of iron/steel. Other qualities are selected and sold on the basis of purity and hardness, but without material analysis. 

As Jacques stated, carbon content in the cutting edges of Japanese swords changed over time. One approach to find the optimum steel may be the fact that carbon content in steel has a peak at 0.78%. At that point, carbon is fully solved in the steel matrix without forming carbides. This leads to a very fine structure and very high hardness which can go up to HRC 67. A higher carbon content does not result in higher hardness.  

Before the upcoming scientific research in the industrial era, there was no knowledge about chemistry and metallurgy, but there was experience. Steel can be "felt" when working with it, and making small samples, hardening and tempering them, and watch how they 'behave' in bending or breaking tests tells a swordsmith all that is necessary to know.