Materials

Introduction

This article shows proper substrate materials in application of TRD coating process to the parts need hardened substrates such as various dies and molds, tools, and wear parts used in material processing as well as machine components.

 

Basic concept in selection

TRD coating can make carbide coatings on any kinds of metals and ceramics containing some amount of carbon.  However, the substrates of the parts coated with TRD should have enough high hardness (compression strength) so as to prevent plastic deformation of the substrates and the resulting cracking in the coatings in use except when TRD application is aimed at improving only resistance to corrosion and oxidation of materials. 

 

Materials have high hardness or can be hardened by heat treating

The materials have high hardness list ceramics and cements, compound of ceramics and metals, including cemented carbides.  Hardness of these materials is higher than that of hardened steels.  The steel for substrates should have some amount of carbon in general so that enough high hardness is obtainable by quench-hardening.  The higher hardness can be attained with the higher carbon content in steels up to 0.8%C very roughly speaking.  The hardness of HRC 40 –45 that usually employed for hot forging dies can be obtained with 0.3% C.  Therefore, it is said that the substrate steels for TRD coating should have carbon content of more than 0.3%.  It is not necessarily correct but allowable as a standard though not quite satisfactorily.

 

Air hardening steels are recommendable

Quench - hardening of substrates is done in TRD coating operation. Quench-hardening of steels can result in distortion: shape deformation and dimensional change, more or less, due to change of crystal phases and inhomogeneous heating and cooling.   The faster cooling in the quenching makes more distortion. Therefore, air hardening steels that can be hardened by slow cooling like as air cooling are highly recommendable for the parts closely tolerated in dimension and shape, rather than water hardening steels and oil hardening steels. TRD coated tools are sometimes harden by additional hardening, post-hardening to realize the maximum hardness of the substrates by using vacuum furnaces that cannot apply the rapid cooling as oil quenching. Therefore, air hardening steels are highly recommendable.

  It the parts can allow very loose dimensional control, water hardening and oil hardening steels can used.  These steels are widely used to TRD application to machine components and even to tools, including metal forming dies actually.

 

Recommendable air hardening steels for TRD

There are two types of air hardening steels, air hardening tool steels and alloyed structural steels.  The air hardening tool steels usually contain fair amounts of alloying elements, chromium, molybdenum, tungsten, vanadium etc. as well as carbon.  These large amount of the alloying elements and carbon produce large amount of particles of carbides in steels, resulting in the smaller dimensional changes during the hardening. Therefore, air hardening high alloyed tool steels are more suitable in comparison with air hardening structural steels with fewer alloying elements and carbon、such as 4140, 4340 etc. 

The typical air hardening tool steels are most of A series, D series, S series, H series, and high speed steels.  However, there are large differences, within these steels, in harden ability, the thickness of parts can be through harden to their center by air cooling. High speed steels are generally poor in hardenability, meaning only relatively thinner tools can be harden thoroughly. 

 

1)  A- type and D –type steels

Among numbers of air hardening tool steels, only some are highly recommendable although other steels also can be used for TRD.  D2 and A2 are most widely used in the world for TRD.  Some tool steels have large amount of vanadium as an alloying element, for example, D7 and A7 have 4 % and 4.75 % of vanadium respectively.  The increased vanadium contents increase amount of particles of MC type carbide in structure.  MC carbide, of which major alloying element is vanadium, is harder than aluminum oxides grits in grinding wheels.  Then use of the high vanadium steels with very poor grindability as well as poor toughness is detrimental rather than worthless as long as TRD coating is applied. 

 

2)  8 % Chromium cold working die steels

New cold working die steels with 8 % chromium (chromium content in D series and A series are 13 % and 5%, respectively), originally developed in Japan, have large profits in application in TRD.  Hardness higher than that  of D2 by HRC 2-3 can be obtained by the high temperature tempering. This is very advantageous for TRD processing as well as the larger hardnability in comparison with D2.  Furthermore, the larger toughness, 2-3 time of D2, and much better machinability provide give large advantages to die making.  The 8% chromium steels have been marketed in the USA industry list DC 53 (Daido Steel – Japanese), CruWear (Crucible), and K340 (Böhler).  However, DC 53 is highly recommendable. Unlike DC 53, the hardening temperatures (austenitizing temperature) can produce the highest quench-hardness for all other steels are higher than the standard TRD temperature for D series, H series, and high speed steels.  This can result in longer delivery time and higher TRD coating cost.

 

3) P/M die steels

Recently lots of powder (particle) metallurgical tool steels have been put into the USA market.  The carbide particles in P/M tool steels are very fine and uniformly distributed in the structures.  Furthermore there is smaller segregation of alloying element within pieces of tools, namely small scattering in chemical composition and distribution of carbide particles. Therefore, P/M tool steels produce less deformation: bend and out-of-roundness, in application of TRD, comparing with those of conventional wrought tool steels.  Also they have better machinability, grindability and toughness.  Therefore, employment of P/M tool steels for TRD coating is gradually increasing in the USA. These include CPM 1V, CPM 3V, CPM 9V, CPM 10V, CPM 15V (Crucible), Vanadis 4, Vanadis 6, Vanadis 10 (Uddeholm) and K190 ISOMATRIX (Böhler).

 However, only some kinds of the P/M tool steels are highly recommendable although other steels also can be used for TRD. Some have 1-3 wt. vanadium as an alloying element, and some have 5% to 15 % vanadium.  Steels with so much vanadium cannot be recommendable due to the reason already discussed.  CPM 1V, CPM 3V and K190 ISOMATRIX are good with lower vanadium content, but their hardening temperature is also higher than the standard TRD coating temperature.  As a conclusion, Vanadis 4  and Vanadis6 are the best choice for TRD.

 

4) High speed steels
High speed steels feature very high hardening temperature (2200ºF, 1200ºC) that is far higher than the optimum treating temperature of ordinary TRD coating furnaces (1830 – 1920ºF, 1000-1025ºC). Therefore, high speed steel tools are usually post-hardened by using a vacuum furnaces after the TRD coating.  This provides not only more chance for the distortion problem but also longer delivery and cost.

Superiority of high speed steels to other tools steels lists higher hardness at room temperature and larger resistance to softening by thermal loading in uses. If the parts are used under the temperature as below as 400ºF, use of  other tool steels as 8% Chromium steels and P/M tool steels, depending on required room temperature hardness, may be smarter.

 

5) P/M high speed steels

The austenitizing temperatures for P/M high speed steels recommended by tool steel makers are usually very high as for the conventional high speed steels.  However, unlike the conventional high speed steels, the P/M high speed steels can be hardened to very high hardness, similar to those by austenitized at high temperature, only by TRD coating operation at the standard TRD temperature, resulting no need for post-hardening, which can produce distortion problems often.  Like as the P/M tool steels, they have advantages of P/M, better machinability, grindability and toughness.  Then, P/M high speed steels are highly recommendable although price of steels is expensive.  The hardness obtainable under standard TRD coating condition ranges HRC 60 – 68 or 69 depending on cobalt contents in steels.  The higher hardness can be achieved with the higher cobalt content.

 

6) Matrix high speed steels

  Matrix high speed steels feature toughness much larger than those of the normal high speed steels, keeping the resistance to softening similar to those of the normal high speed steels do.  Then highly recommendable to tools foe cold forging, warm forging and hot forging. Unfortunately, there are no steels widely marketed in the USA industry.  However, Japanese steels such as YXR 3, YXR 33, MDS1, MDS3,and MH 85 are available now in the USA. 

 

7) Hot working die steels

Tools used under high temperature should have resistance to softening by heat. For these tools, H series steels are recommendable.  H steels with tungsten and cobalt have larger resistance to softening but still their resistance to softening are inferior to those of high speed steels.  There are some modified steels as for hardenbility and toughness recently marketed in the USA.  There are recommendable.

 

Cemented carbides

Cemented carbides (Compound of WC, TiC, TaC as carbides and cobalt as binder) are most widely used for TRD among ceramics and cements. Tribological property of cemented carbides is much superior to those of any tool steels and high speed steels but still inferior to those of TRD coatings. Therefore TRD coating is actually applied to cemented carbide tools very often. However, use of cemented carbides as substrates should be limited to the following cases: 

1. Mechanical loading in use is so high that steel substrates are subjected to plastic deformation while using even if their hardness was achieved to very high like as HRC 68.

2.  Parts need very tightly tolerated like as ±5 µm and the tolerance cannot be insured by steel substrates.

 

Summary

Attached table is summary of recommendable materials in application classified under loading condition in use.

There is quite big difference in cost of substrate materials.  Less expensive materials are recommendable usually.  However, selection of substrate materials should be done under well consideration of the total cost including life improvement of the parts, needed repair works, down time for repair etc.

Working Temperature	Requirement	Mechanical Loading in Usage
		Small	Standard	Slightly Large	Large	Larger	Extremely Large
	Recommended Substrate hardness HRC	40-50	50-58	59-61	61-63	65-68	above 90
Near Room Temperature(Cold)	Standard	Structual Steel * Cast Iron*	Structual Steel W1 O1 A2 D2 8% Cr Die Steel	8% Cr Die Steel	M2	High Speed Steel with 8-12% Co	Cemented Carbide
	Toughness				P/M High Speed Matrix High Speed	Steel with 8-12% Cr	Carbide
	Tight Dimentional Tolerance		PM Die Steel		P/M High Speed Steel	P/M Highspeed steels with 8-12% Co	Cemented Carbide
Working Temperature	Requirement	Thermal Loading in Usage
		Small	Standard	Slightly Large	Large	Larger	Extremey Large
Warm	Standard		H12 H13	H10	H19 H21 M2	High Speed Steels with CO	Cemented Carbide
	Toughness required		H11		H19 H12 Matrix High Speed	P/M High SpeedSteel with CO	High CO cementedCarbide
Hot	Standard	6G 6F2	H12 H13		M2		Cemented Carbide
	Toughness required		H11	Matrix High Speed Steel	Matrix High Speed Steel		High CO cementedCarbide

Remarks: *Some of these are water or oil hardening and can make large distortion  
       
Recommended Materials in each category     
Structural Steel-52100,1045,4140,4135.4340.etc.    
8% Cr Die Steel - DC53 (Daido)      
P/M Die Steel - Vanadis 4, Vanadis 6 (Uddeholm), CPM10V (Crucible)   
Matrix high Speed steels-YXR3, YXR 33(Hitachi), MH85 (Daido)   
P/M High Speed Steel-Vanadis 23 (Uddeholm), CPM M4 (Crucible)   
P/M High Speed Steel with 8-12%CO- Vanadis 30, 60 (Uddeholm), CPM Rex 45, CPM Rex76, CPM Rex 121 (Crucible), Micro-Melt M30, 
Micro-Melt HS30, Micro Melt HS76 (Carpenter), S390 Isomatrix, S590 Isomatrix (Bohler)  

TeikuroTRD has emphasized material selection is one of the key factor for successful TRD application.  We provide free consultation for substrate material selection.

 

Characteristics of Surface Treatments

		Tool Steel	Cemented Carbides	Teikuro TRD	CVD TiC , TiCN	PVD TiN , TiCN	Nitriding	Chrome Plating
Best A-B-C-D-E Worst
Process	Surface Material	Steel	Carbide + Cobalt	Carbide	Carbide, Carbonitride Nitride	Nitride, Carbonitride Carbide	Cabonitride	Chrome
	Thickness of coating ( um )	0	0	2 to 15	2 to 15	2 to 10	0 to 10 + 0.1 to 1mm	10 to 50
	Treating Temperature	-	-	High	High	Low	Low	Room
	Throwing Power	-	-	Excellent	Good	Poor	Good to Excellent	Good
	Possible Distortion	-	-	Larger	Larger	Medium to Small	Medium to Small	None
Surface	Hardness at Low Temperature	E	D	A	A	A	D	D
	Hardness at High Temperature	D	B	A	A	A	C	E
	Resistance to Tempering	E	C	A	A	A	D	E
	Resistance to Abrasive Wear	E	B	A	A	A	C	C
	Resistance to Adhesive Wear	E	B	A	A	A	D	C
	Resistance to Galling	E	B	A	A	A	D	C
	Deformation Cracking	-	-	C	C	C	B	D
	Adhesion Strength	-	-	A	A	C	A	D
Coated Material	Fatigue Strength	C	-	B-D	B-D	B-C	A	D
	Heat Checking	C	-	B	B	B	B-D	C

 

 

Specification and Characteristic of Typical Substrate Materials

HRC Hardness After Coating	Material Types	Major Alloying Element	Making Process	Major Standard or Brand Name	Characteristics
Above 69	Cemented Carbide	80%-95% WC + Cobalt	Sintering of powder		Brittle, Not Machinable
65-69	High Speed Steel with High Cobalt	C-W-Mo-CR-V-CO	Casting, then Forging	M42, M43	Relatively Brittle, Higher Hot Hardness, Higher resistance to Temper-Softening
			HiP , then Forging	Vanadis 30, 60 Micro-Melt M42	Higher Hot Hardness, Higher resistance to Temper-Softening, Relatively Tough, Less Distortion in Heat Treatment and Coating
61-65	High Speed Steel with No or Low Cobalt	C-W-Mo-CR-V- (CO)	Casting, then Forging	M2, M4, M7, M35	High Hot Hardness High Resistance to Temper-Softening
			HiP then Forging	Vanadis 23, CPM M4	High Hot hardness, High Resistance to Temper-Softening. Less Distortion in Heat treatment and Coating
57-61	Matrix High Speed Steel	C-W-Mo-Cr-V-(Co)	Casting, then Forging	MH-85, YXR3 YXM33	Higher Toughness, Resistance to Heat Checking, then High Speed Steel
	Innovative Cold Working Die Steel	C-Cr-<W>-<Mo>-<V>	Casting, then Forging	DC53, Cruwear	Higher Hardness, Higher Toughness and easy to Machine than D2
		C-Cr-<W>-<Mo>-V	HiP , then Forging	Vanadis 4,10 CPM10V, 15V	Less Distortion in Heat Treatment, and High Temperature Coating. Worse Grindability with High Vanadium Content.
	Standard Cold Working Die Steel	C-Cr-<W>-<Mo>-<V>	Casting, then forging	A2, D2	Average in various properties

High hardness can be used as measuring rule for lower possibility of substrate deformation
< > : Small amount, less than 1 %

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Specific application data, detailing the process and benefits of particular uses for the TRD process.

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Summary of recommended materials based on mechanical or thermal usage. 

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Teikuro’s TRD coating process can be used in virtually any industries where tooling problems occur.

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