With the diffusion of nitrogen into the steel surface, a volume change will occur, which means a size change in the form of growth.. With gas nitriding, and consider-ing nitridconsider-i
Trang 1When a piece of steel is austeniti zed and cooled at various rates (as c an occur due to section al thickne ss changes) , various structures can result The structure of the austeni te phase has the smal lest volume , and the untemper ed marten site pha se has the large st phase
If there are mixe d phases, an y residual austeni te wi ll transform to martensit e over time or with the ap plication of he at This will cause a dimension al change in the steel
With the diffusion of nitrogen into the steel surface, a volume change will occur, which means a size change in the form of growth The amount of growth that will take place will be determined by the thickness of the formed case The thickness of the formed compound layer will also contribute to the amount of growth With gas nitriding, and consider-ing nitridconsider-ing steel, the thickness of the compound layer is generally 10% of the total case thickness Do not be confused by this to mean the effective case It is the total case With the ion nitride procedure, the thickness of the compound layer can be controlled by the gas ratios selected for the process, which ultimately means the growth can be controlled more effectively There will always be a growth, no matter what process method is chosen The growth will also
be uniform in all directions Another method of ensuring dimensional stability is to subject the steel to a cryogenic treatment followed by a final temper, followed by the final machine and then the nitride procedure The cryogenic treatment will ensure a complete phase change, which means any residual retained austenite will be transformed to untempered martensite This means that no further phase transformation will occur and will thus ensure dimensional stability of the part
FERRITIC NITROCARBURIZING
8.19 INTRODUCTION
FNC is a low-te mperatu re proc ess that is process ed in the ferr ite region of the iron–ca rbo n equilib rium diagra m at a pro cess tempe ratur e of approxim ately 580 8 C (1075 8 F) The object -ive of the process is to form both carbide s and nitrides in the imm ediate surfa ce of the steel The process is usu ally ap plied to low-car bon and low-al loy steels to enhance the surfa ce charact eristic s in terms of hardn ess an d corrosi on resi stance In ad dition to this , the surface is furth er enhan ced by delibe rately ox idizing the surfa ce to pro duce a corrosi on-res istant surfa ce oxide barri er to the steel The process has gained a great deal of populari ty during the past 5 to 10 years (Figur e 8.23)
The process is diffusional in nature an d intr oduc es both nitro gen and carbon into the steel surfa ce while the steel is in the ferr ite pha se with respect to the tempe ratur e Nitrogen is solubl e in iron at the tempe rature range of 315 8 C (6008 F) and upwar d Carbon is also solubl e
in iron at a tempe ratur e higher than 370 8 C (700 8F) These elem ents are soluble in a soli d solut ion of iron Genera lly the pr ocess oc curs at a temperatur e range of 537 (1000) to 600 8C (1100 8 F) The diffused elem ents will form a surface co mpound layer in the steel which produces good wear and fatigue properties in the steel surface Below the compound layer
is the diffused nitrogen solid solution in a diffusion layer In other words, the case formation
is very simila r to that of nitriding (Figur e 8.24)
The process started life as a cyanide-based salt bath process around the late 1940s and components such as high-speed auto components (including gears, cams, crankshafts, valves) were processed It was used primarily as an antiscuffing treatment This process was also used on cast iron components for an improvement in antiscuffing resistance During the 1950s, investigatory work was conducted in the U.K into gaseous methods of FNC [15]
Trang 2When a piece of steel is austeniti zed and cooled at various rates (as c an occur due to section al thickne ss changes) , various structures can result The structure of the austeni te phase has the smal lest volume , and the untemper ed marten site pha se has the large st phase
If there are mixe d phases, an y residual austeni te wi ll transform to martensit e over time or with the ap plication of he at This will cause a dimension al change in the steel
With the diffusion of nitrogen into the steel surface, a volume change will occur, which means a size change in the form of growth The amount of growth that will take place will be determined by the thickness of the formed case The thickness of the formed compound layer will also contribute to the amount of growth With gas nitriding, and consider-ing nitridconsider-ing steel, the thickness of the compound layer is generally 10% of the total case thickness Do not be confused by this to mean the effective case It is the total case With the ion nitride procedure, the thickness of the compound layer can be controlled by the gas ratios selected for the process, which ultimately means the growth can be controlled more effectively There will always be a growth, no matter what process method is chosen The growth will also
be uniform in all directions Another method of ensuring dimensional stability is to subject the steel to a cryogenic treatment followed by a final temper, followed by the final machine and then the nitride procedure The cryogenic treatment will ensure a complete phase change, which means any residual retained austenite will be transformed to untempered martensite This means that no further phase transformation will occur and will thus ensure dimensional stability of the part
FERRITIC NITROCARBURIZING
8.19 INTRODUCTION
FNC is a low-te mperatu re proc ess that is process ed in the ferr ite region of the iron–ca rbo n equilib rium diagra m at a pro cess tempe ratur e of approxim ately 580 8 C (1075 8 F) The object -ive of the process is to form both carbide s and nitrides in the imm ediate surfa ce of the steel The process is usu ally ap plied to low-car bon and low-al loy steels to enhance the surfa ce charact eristic s in terms of hardn ess an d corrosi on resi stance In ad dition to this , the surface is furth er enhan ced by delibe rately ox idizing the surfa ce to pro duce a corrosi on-res istant surfa ce oxide barri er to the steel The process has gained a great deal of populari ty during the past 5 to 10 years (Figur e 8.23)
The process is diffusional in nature an d intr oduc es both nitro gen and carbon into the steel surfa ce while the steel is in the ferr ite pha se with respect to the tempe ratur e Nitrogen is solubl e in iron at the tempe rature range of 315 8 C (6008 F) and upwar d Carbon is also solubl e
in iron at a tempe ratur e higher than 370 8 C (700 8F) These elem ents are soluble in a soli d solut ion of iron Genera lly the pr ocess oc curs at a temperatur e range of 537 (1000) to 600 8C (1100 8 F) The diffused elem ents will form a surface co mpound layer in the steel which produces good wear and fatigue properties in the steel surface Below the compound layer
is the diffused nitrogen solid solution in a diffusion layer In other words, the case formation
is very simila r to that of nitriding (Figur e 8.24)
The process started life as a cyanide-based salt bath process around the late 1940s and components such as high-speed auto components (including gears, cams, crankshafts, valves) were processed It was used primarily as an antiscuffing treatment This process was also used on cast iron components for an improvement in antiscuffing resistance During the 1950s, investigatory work was conducted in the U.K into gaseous methods of FNC [15]
Trang 39 Quenching and Quenching Technology
Hans M Tensi, Anton Stich, and George E Totten CONTENTS
9.1 Introduction 540
9.2 Metallurgical Transformation Behavior during Quenching 541
9.2.1 Influence of Cooling Rate 541
9.2.2 Influence of Carbon Concentration 544
9.2.3 Influence of Alloying Elements 544
9.2.4 Influence of Stresses 548
9.3 Quenching Processes 549
9.4 Wetting Kinematics 551
9.5 Determination of Cooling Characteristics 553
9.5.1 Acquisition of Cooling Curves with Thermocouples 553
9.5.2 Measurement of Wetting Kinematics 558
9.5.2.1 Conductance Measurement 558
9.5.2.2 Temperature Measurement 559
9.6 Quenching as a Heat Transfer Problem 560
9.6.1 Heat Transfer in a Solid 560
9.6.2 Heat Transfer across the Surface of a Body 562
9.7 Process Variables Affecting Cooling Behavior and Heat Transfer 567
9.7.1 Immersion Quenching 567
9.7.1.1 Bath Temperature 567
9.7.1.2 Effect of Agitation 568
9.7.1.3 Effect of Quenchant Selection 569
9.7.1.4 Surface Oxidation and Roughness Effects 569
9.7.1.5 Effect of Cross-Section Size on Cooling 571
9.7.1.6 Effects of Cooling Edge Geometry 573
9.7.1.7 Effects of Steel Composition 574
9.7.2 Spray Quenching 575
9.7.3 Gas Quenching 578
9.7.4 Intensive Quenching 583
9.8 Property Prediction Methods 589
9.8.1 Potential Limitations to Hardness Prediction 590
9.8.2 Grossmann H-Values 591
9.8.3 The QTA Method 594
9.8.4 Correlation between Hardness and Wetting Kinematics 596
9.8.5 Computer-Based Calculation of Hardness Profile 599
List of Symbols 601
References 602
Trang 40 10 20 30 40 50
− 200 0
Mf
400 600 800
0 20 30 40 50 60 70
Carbon content in wt%
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Carbon content wt%
Carbon content in wt%
(c) (b)
(a)
x x
x x
x x
x x x
% Martensite
99.9%
90.0%
80.0%
c
Ni MnSi CrSi CrNiMo CrNi Mo CrMo Cr
Steels
Maximum hardness according to Burns, Moore and Archer Hardness at different % of marten-site according to Hodge and Orehaski
FIGURE 9.4 Effect of carbon concentration on (a) hardness for structures with different martensite content; (b) temperature for starting and completing the martensite formation Msand Mf; (c) retained austenite
Trang 50.1 0 100
200
300
400
500
600
700
800
900
1,000
Ck45 0.44% C–0.66% Mn (SAE 1042) Composition: 0.44% C–0.66% MN–0.22% P–
A Area of austenite formation
F Area of ferrite formation
P Area of pearlite formation
Zw Area of intermediate structure (bainite formation)
M Area of martensite formation
Austenitizing temperature 8808C
(Holding time 3 min) quench in 2 min
1 s
(a)
min
h
10
1 10
1 10
100 1,000
Ac3
A
M
Ms
Ac1
60 40 50
50 30
70 75 25 80 10 10
2 20 5 17
1 3 P F
270
224 274 274 318 533 548
Time
Hardness in HRC or HV 1,2—Compostion in %
Ac1 = 7458C
Ms = 3558C
Zw
0 100
200
300
400
500
600
700
800
900
1,000
Austenitizing temperature 8408C
(Holding time 8 min) quench in 3 min
41Cr4 (SAE 5140) Composition: 0.44% C–0.80% Mn–0.22% Si–0.030% P–
0.023% S–1.04% Cr–0.17% Cu–0.04% Mo–0.26% Ni–
<0.01% V austenitized at 8408C (15448F)
Hardness in HRC or HV 2,3—Compostion in %
Ac1 = 7458C
Ms = 3558C
1
1
1 10 100
10 100 1,000 10,000
10 s
min
h
(b)
Time
A Area of austenite formation
F Area of ferrite formation
P Area of Pearlite formation
Zw Area of intermediate structure (bainite formation)
M Area of martensite formation 180
230 20
27 36 34 38 44 52 54 60
Ac3
Ac1 40 40
70 80 20 2 4 3
3 Zw 5
P A
M
Ms
F 50
60 70 60 25
60 60
FIGURE 9.5 Influence of allowing elements, here chromium, on the transformation of subcooled austenite described according to CCT diagrams of (a) a 1040 steel with about 0.15 wt% Cr (German grade Ck 45) and (b) a 5140 steel with about 1 wt% Cr (German grade 41 Cr 4), and
Continued
546 Steel Heat Treatment: Metallurgy and Technologies