One Minute Mentor: Time-Temperature-Transformation Diagram
October 31, 2017 Source: ASM International
Traditionally, carburization is carried out at high temperature to maximize the solubility and the rate of interstitial solute diffusion.
Continue readingOne Minute Mentor: High-Temperature Nitriding for Deeper Case
March 2, 2017 Source:
High-temperature nitriding or solution nitriding at temperatures of 2010 ± 90°F is a new treatment available to produce cases with nitrogen concentrations in the range of 0.4 to 0.9 wt%, which is low compared to plasma nitriding treatments at low temperature (up to 16.9wt% N), but the resulting case is up to 100 times deeper (2 mm, or 0.08 in.). The lower nitrogen concentration implies a lower surface hardness (360 HV for solution nitriding of austenitic steels).
The graph shows a time-temperature transformation (TTT) curve for the precipitation of Cr2N, from which it can be estimated that, to avoid CrN precipitation, the temperature must drop from 2010°F to below 1650°F in approximately 20 seconds. That is feasible only in relatively thin parts (large area/volume ratio) that cool sufficiently to cope with the TTT curve time limitations to avoid precipitates, and only for a small number of parts at a time.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Nitriding of Stainless Steels ⇒ Thermochemical Nitriding Treatments
High-temperature nitriding or solution nitriding at temperatures of 2010 ± 90°F is a new treatment available to produce cases with nitrogen concentrations in the range of 0.4 to 0.9 wt%, which is low compared to plasma nitriding treatments at low temperature (up to 16.9wt% N), but the resulting case is up to 100 times deeper (2 mm, or 0.08 in.).
Continue readingOne Minute Mentor: Removing the Passive Layer in Low-Temperature Gas Nitriding
January 27, 2017 Source:
The recent advent of low-temperature gas nitriding (for the production of the S-phase in stainless steel) as opposed to regular high-temperature (above 550 °C) gas nitriding (of regular steel) had to find a way to remove the passive layer in this low-temperature condition.
Patent EP 1707646 B1 (2005): The process set forth in this patent uses a gaseous carbon donor compound (such as acetylene, ethylene, propane, butane, or carbon monoxide) with ammonia at temperatures greater than 570°F. It is carried out in a reactor vessel made of metal such as iron, nickel, cobalt, etc. The catalytic action of the reactor vessel will generate HCN. The HCN will depassivate the stainless steel surface through the following reactions:
2NH + C2H2 → 2HCN + 3H2
Cr2O3 + 6HCN → 2Cr(CN)3 + 3H2O
The carbon and nitrogen in 2Cr(CN)3 diffuse inward and form the nitrocarburized case, leaving a clean surface. The dewpoint must be kept below 40 °F to keep the metal surface being treated from repassivating, which means that the flow of gas must be continuous to eject excessive water formed during the depassivation and nitrocarburizing reaction. After the depassivation step, ordinary gas nitriding with NH3 + H2 + N2 atmosphere can be carried out as desired. The exhaust gas is burned and decomposed at the exit as nitrogen and CO2.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Nitriding of Stainless Steels ⇒ Thermochemical Nitriding Treatments
The recent advent of low-temperature gas nitriding (for the production of the S-phase in stainless steel) as opposed to regular high-temperature (above 550 °C) gas nitriding (of regular steel) had to find a way to remove the passive layer in this low-temperature condition.
Continue readingOne Minute Mentor: Duplex Stainless Steels
September 1, 2016 Source:
Complex compounds of iron, chromium, and molybdenum are highly detrimental to toughness and corrosion resistance, particularly the resistance to localized chloride attack. The graph shows the time-temperature exposures required for a 50% reduction in impact strength due to isothermal precipitation of undesirable secondary phases for various duplex stainless steels.
The kinetics of σ/χ precipitation is greatest in the range of about 1500 to 1700°F. The time required to degrade properties depends on the temperature and the alloy content. The increased N content of modern duplex stainless steels significantly decreases the rate of the σ/χ precipitation reactions making it possible to weld these grades without loss of properties due to formation of intermetallic phases. Once intermetallic phases have formed, the only means of removing them and restoring properties is a full solution anneal followed by rapid cooling.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Heat Treating of Austenitic and Duplex Stainless Steels ⇒ Duplex Stainless Steels
Complex compounds of iron, chromium, and molybdenum are highly detrimental to toughness and corrosion resistance, particularly the resistance to localized chloride attack.
Continue readingOne Minute Mentor: Time-Temperature Sensitization Curves for Austenitic Stainless Steel
June 17, 2016 Source:
The graph shows a Time-Temperature-Sensitization diagram (TTS) for 304 austenitic stainless steel with various carbon contents. Decreasing carbon content significantly delays the formation of chromium carbides and the onset of intergranular corrosion. In low carbon grades, defined as those with carbon contents below 0.030%, an exposure of about eight hours is necessary before sensitization occurs.
Low-carbon austenitics are intermediate in tendency to precipitate chromium carbides to the stabilized and unstabilized grades. The lower carbon content in an austenitic stainless steel will slow the kinetics of carbide precipitation, which allows longer time at temperature before the onset of sensitization, as shown in the graph.
Carbon contents of 0.03% and lower allow short excursions of several hours into the sensitizing temperature range without loss of corrosion resistance. Carbon in austenite can be retained in supersaturation for extended periods of time. This is why austenitic L grades do not sensitize even though they are slightly supersaturated. Sensitization occurs at higher levels of carbon by prolonged heating at 600 to 850°C.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Heat Treating of Austenitic and Duplex Stainless Steels ⇒ Annealing
The graph shows a Time-Temperature-Sensitization diagram (TTS) for 304 austenitic stainless steel with various carbon contents.
Continue readingOne Minute Mentor: Stabilization
May 20, 2016 Source:
Stabilization is generally considered to be the simple gettering of carbon and nitrogen by a suitable carbide former such as titanium or niobium. Stabilization is essential for ferritic stainless steels to avoid the precipitation of grain boundary carbides.
For example, if carbon and nitrogen are kept as low as possible and a stoichiometric amount of titanium is available (i.e., greater than about four times the carbon plus nitrogen), then essentially all carbon is removed from solution (below 2280°F). However, the stabilization formula in various specifications is more than four times the carbon plus nitrogen, because experiments show that sometimes understabilization occurs. This is due to the influence of oxygen and sulfur having already formed compounds with the titanium, and to less-important kinetic factors that have prevented TiC formation.
The graph shows the time-temperature-transformation (TTT) curve for an unstabilized 430-type alloy with carbon plus nitrogen of 0.08%.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Heat Treating of Ferritic Stainless Steels ⇒ Stabilization
Stabilization is generally considered to be the simple gettering of carbon and nitrogen by a suitable carbide former such as titanium or niobium. Stabilization is essential for ferritic stainless steels to avoid the precipitation of grain boundary carbides.
Continue readingOne Minute Mentor: Stabilization of Ferritic Stainless Steels
Source:
Stabilization is generally considered to be the simple gettering of carbon and nitrogen by a suitable carbide former such as titanium or niobium. Stabilization is essential for ferritic stainless steels to avoid the precipitation of grain boundary carbides.
For example, if carbon and nitrogen are kept as low as possible and a stoichiometric amount of titanium is available (i.e., greater than about four times the carbon plus nitrogen), then essentially all carbon is removed from solution (below 2280°F). However, the stabilization formula in various specifications is more than four times the carbon plus nitrogen, because experiments show that sometimes understabilization occurs. This is due to the influence of oxygen and sulfur having already formed compounds with the titanium, and to less-important kinetic factors that have prevented TiC formation.
The graph shows the time-temperature-transformation (TTT) curve for an unstabilized 430-type alloy with carbon plus nitrogen of 0.08%.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Heat Treating of Ferritic Stainless Steels ⇒ Stabilization
Stabilization is generally considered to be the simple gettering of carbon and nitrogen by a suitable carbide former such as titanium or niobium. Stabilization is essential for ferritic stainless steels to avoid the precipitation of grain boundary carbides.
Continue readingOne Minute Mentor: Heat Treatment of a Premium Grade H13 Die
February 4, 2016 Source:
The graph shows a continuous cooling test in which the austenitizing temperature was approximately 1040°C (1900°F). It shows the effect of intermediate cooling rates on the microstructure and toughness of H13 tool steel austenitized at 1075°C (1970°F). (a) Continuous cooling transformation (CCT diagram) shows variation in microstructure with varying cooling rates. (b) Variation in toughness as a function of carbides ejected from austenite. Hardness was maintained at a constant value.
Hot-work die steels containing 5% Cr are normally used (H13 is the most common in the United States) because they have:
• A good combination of toughness and strength for safety and service
• Good hardenability in thick sections
• Good hot hardness and strength to resist cyclic stressing at operating temperatures up to 400 °C (750 °F)
• Good resistance to tempering during service that gradually lowers the surface hardness and the fatigue strength
Carbide precipitation, other than that produced during tempering, is known to be deleterious to the fatigue life of H13 tool steel. Fast quenching rates required to suppress intermediate precipitation of carbides (such as proeutectoid grain-boundary carbide, pearlite, or bainite) also produce large thermal gradients within the tool. Fast quenching rates can cause distortion or even cracking. The larger the tool, the greater the risk of cracking.
Volume 4D, Heat Treating of Irons and Steels ⇒ Tool Steels ⇒ Heat Treating of Hot-Work Tool Steels ⇒ Examples of Heat-Treating Procedure for Hot-Work Tools
The graph shows a continuous cooling test in which the austenitizing temperature was approximately 1040°C (1900°F).
Continue readingOne Minute Mentor: Isothermal Transformation Diagram for Tool Steels
October 23, 2015 Source: ASM International
The graph shows the isothermal transformation diagram for the steel AISI 1075/DIN EN C75. Cooling curves show three process cycles for molten metal quenching in the time-temperature-transformation plots.
Continue readingOne Minute Mentor: Steps for Hardening of Tools as a Function of Time, Temperature, and Phase Transformation
October 9, 2015 Source:
Steps for Hardening of Tools as a Function of Time, Temperature, and Phase Transformation
After hot forming, tool steel usually is submitted to heat treatment processes that may be split into two major steps. The first step (shown last week) is done to produce a low-hardness condition. The second step involves this homogeneous microstructure being converted to a higher-hardness condition, usually by martensite formation during the hardening and tempering heat treatments.
In a graphical form, the graph shows the typical processing and heat treatment sequences for hardening of tool steels, as a function of time, temperature, and phase transformation.
The graph shows plots of temperature versus time showing sequence of operations required to produce tool steels. Hardening heat treatment. L, liquid; A, austenite; C, cementite; F, ferrite; Ms, temperature at which martensite starts to form on cooling; RT, room temperature.
Volume 4D, Heat Treating of Irons and Steels -> Heat Treating of Carbon and Low-Alloy Steels -> Introduction to Heat Treating of Tool Steels -> Heat Treatment Applied to Tool Steels
After hot forming, tool steel usually is submitted to heat treatment processes that may be split into two major steps.
Continue reading