When 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 nitriding 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]. ß 2006 by Taylor & Francis Group, LLC. When 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 nitriding 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]. ß 2006 by Taylor & Francis Group, LLC. 9 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 Character istics 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 Beha vior 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 Kinem atics 596 9.8.5 Computer-Based Calculation of Hardness Profile 599 List of Symbols 601 References 602 ß 2006 by Taylor & Francis Group, LLC. 0 0 10 20 30 40 50 −200 0 200 M s M f 400 600 800 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0 20 30 40 50 60 70 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 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% Hardnes in HRC Temperature in 8C Retained austenite in vol% (c) (b) (a) x x x x x x x x x x % Martensite 99.9% 95.9% 90.0% 80.0% 50.0% 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 M s and M f ; (c) retained austenite. Quenching and Quenching Technology 545 ß 2006 by Taylor & Francis Group, LLC. 0.1 0 100 200 Temperature in 8C 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– 0.029% S−0.15% Cr–0.02% V austenitized at 8808C (16168F) 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 10 110 110 100 1,000 10 2 10 3 10 4 10 5 A c3 A M M s A c1 60 40 50 50 30 70 75 25 80 10 10 2 20 5 17 1 3 P F 70 179 270 224274274 318 533 548 Time Hardness in HRC or HV 1,2—Compostion in % A c1 = 7458C A c3 = 7908C M s = 3558C Zw 0 100 200 Temperature in 8C 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 % A c1 = 7458C A c3 = 7908C M s = 3558C 1 1 110 100 10 100 1,000 10,000 10 s min h (b) 10 2 10 3 10 4 10 5 10 6 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 2027 36 34 3844 52 54 60 A c3 A c1 40 40 70 30 80 20 2 4 6 3 3 Zw 5 P A M M s 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 ß 2006 by Taylor & Francis Group, LLC. . a 5140 steel with about 1 wt% Cr (German grade 41 Cr 4), and Continued 546 Steel Heat Treatment: Metallurgy and Technologies ß 2006 by Taylor & Francis Group, LLC. . austenitized at 8408C (15448F) Hardness in HRC or HV 2,3—Compostion in % A c1 = 7458C A c3 = 7908C M s = 3558C 1 1 110 100 10 100 1,000 10, 000 10 s min h (b) 10 2 10 3 10 4 10 5 10 6 Time A. 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 nitriding steel,