CTS BD1N is a nitrogen-bearing martensitic stainless steel developed by Carpenter Technology as an advancement over their CTS BD1 steel.
The nitrogen addition gives CTS BD1N better corrosion resistance compared to CTS BD1 steel.
It also enables greater hardness without reducing toughness or grindability.
In contrast, CPM SPY27 is a relatively new steel that was developed jointly by Crucible and Spyderco as an exclusive steel for Spyderco knives.
SPY27 has significantly higher toughness compared to CTS BD1N, which is a critical property for knives affecting strength and chip resistance.
SPY27 contains additions like cobalt and niobium to enhance properties compared to Crucible’s S30V, S35VN, and S45VN alloys.
The niobium carbides improve edge retention while cobalt increases hardness.
SPY27 steel also has 0.11% nitrogen in its composition, however, the nitrogen has a greater effect in CTS BD1N compared to SPY27 due to differences in the production process.
In SPY27 steel the Nitrogen is not actually in solution as in CTS BD1N rather it is added to promote the formation of vanadium and niobium carbide.
CTS BD1N vs SPY27 Steel
| Property | CTS BD1N Steel | SPY27 Steel |
|---|---|---|
| Hardness | Up to HRC 63 | Up to HRC 64 |
| Edge Retention | Moderate, from matrix and chromium carbides | Very good, from vanadium and niobium carbides |
| Ease of Sharpening | Easier due to softer chromium carbides | More difficult due to hard vanadium and niobium carbides |
| Toughness | Lower than SPY27 Steel | Higher than CTS BD1N steel |
| Corrosion Resistance | Very good from nitrogen addition | Good but lower than BD1N, similar to S30V |
Hardness
CTS BD1N steel can achieve a hardness of up to HRC 63, while SPY27 steel can reach a hardness of approximately 64 HRC.
But actual SPY27 knives will mostly be heat treated to a hardness of 60-62 HRC as evidenced by Shawn in testing of SPY27 Para 3 LW Hardness.
The hardness of CTS BD1N steel comes from its high carbon content (0.85-0.95%) along with the addition of 0.1-0.15% nitrogen.
The nitrogen helps increase hardness without negatively affecting corrosion resistance.
Higher nitrogen content leads to more nitrogen in solution after austenitizing, which correlates with higher resulting hardness.
SPY27 steel similarly relies on its carbon content (1.1%) for hardness, along with additions of chromium, vanadium, niobium, and cobalt.
The carbon content in SPY27 is balanced to result in a hardness similar to S30V and S35VN steels, which can also reach 64 HRC.
Edge Retention
SPY27 steel has better edge retention and wear resistance compared to CTS BD1N steel for knife applications.
SPY27 knife steel has moderately better edge retention than S35VN steel, which itself is quite high. SPY27 steel edge retention is attributed to its carbide content and hardness.
It has niobium and vanadium carbides which are very hard and contribute significantly to wear resistance.
The nitrogen in SPY27 promotes the formation of vanadium and niobium carbides over chromium carbides.
This improves wear resistance through a higher amount of hard MC carbides.
CTS BD1N steel wear resistance is derived primarily from the matrix and chromium carbides.
With lower hardness and lack of specialty carbides, it does not exceed SPY27 edge retention and wear resistance.
SPY27 steel higher hardness, niobium, and vanadium carbides give it an advantage here over CTS BD1N blade steel.
These features of SPY27 result in it having better edge retention and wear resistance.
Ease of Sharpening
CTS BD1N should be easier to sharpen and grind over SPY27 due to its reliance on softer chromium carbides rather than hard niobium and vanadium carbides.
SPY27 has a similar carbide content to S35VN and S45VN steels. These steels are relatively difficult to sharpen because of their high volume of hard MC carbides (niobium and vanadium).
In contrast, CTS BD1N is a simple steel without specialty carbides. Its primary abrasion resistance comes from the chromium carbides.
Chromium carbides are noted to be softer and easier to grind away compared to niobium/vanadium carbides.
Toughness
SPY27 steel has superior toughness compared to CTS BD1N steel for knife applications.
SPY27 has a quantitative toughness rating of around 5 out of 10, while CTS BD1N is significantly lower at 3.5 out of 10.
The reasons for SPY27 higher toughness are its lower carbide content particularly the reduction of Vanadium and the addition of niobium to refine the carbide structure.
Its lower carbon compared to steels like S35VN also contributes to better toughness.
In contrast, in CTS BD1N the vanadium in BD1 was replaced by nitrogen in BD1N, which produces more alloying atoms for increased toughness.
And its higher carbide content leads to lower toughness according to the principles outlined.
For knives, edge toughness is a critical property impacting strength and resistance to chipping.
The significantly higher toughness of SPY27 at 5/10 versus CTS BD1N at 3.5/10 makes SPY27 the better choice for optimizing this property.
Corrosion Resistance
CTS BD1N steel has better corrosion resistance compared to SPY27 steel, due to the nitrogen addition in CTS BD1N.
The 0.1-0.15% nitrogen addition in CTS BD1N helps increase hardness without negatively affecting corrosion resistance.
Nitrogen has less tendency to form nitrides with chromium compared to carbon’s tendency to form chromium carbides.
This means more chromium can be retained in solution when nitrogen is added instead of carbon.
Chromium in solution strongly influences positively the corrosion resistance of stainless steel.
Carpenter Technology indicates that the nitrogen addition gives CTS BD1N a small boost to corrosion resistance compared to a standard 440C-type stainless steel.
In contrast, the nitrogen present in SPY27 facilitates vanadium and niobium carbide formation over chromium carbides.
This leaves more chromium available in the matrix for enhanced corrosion resistance.
Its corrosion resistance is estimated to be similar to S30V and S35VN based on chromium in solution.
Final Thoughts on CTS BD1N vs SPY27 Steel
I would generally recommend SPY27 steel over CTS BD1N steel for most knife applications.
I have used extensively my SPY27 Para 3 knife but only limited testing experience with CTS BD1N steel.
From my perspective, SPY27 knife steel is an impressive performer that leverages the qualities of its predecessor steel S30V quite well.
When comparing CTS BD1N and SPY27 side-by-side, CTS BD1N does have advantages in corrosion resistance and ease of sharpening.
However, in real-world use, these benefits are not very noticeable. The corrosion resistance of SPY27 is already quite good, and its sharpenability is not poor.
On the other hand, SPY27 has clear edges when it comes to hardness, edge retention, and especially toughness.
These attributes have a more substantial impact on how a knife performs and holds up during daily carry and use.
Having used SPY27 extensively without issue, and only having limited time with CTS BD1N, my recommendation would be to opt for SPY27 for most knife uses.
Its well-balanced qualities and proven performance as an exclusive Spyderco steel make it a great choice that I’m confident will satisfy most users.
CTS BD1N is not a bad steel, but SPY27 builds on the solid foundation of S30V to deliver a superior all-around package.