# Comprehensive Exam Notes: Heat Treatment of Steels ## 1.0 Quenching Media and Processes Quenching, the rapid cooling of a workpiece, is a critical step in heat treatment used to induce specific metallurgical transformations, most notably the formation of martensite to achieve high hardness. The substance used for this rapid cooling is known as a quenchant. The selection of an appropriate quenchant is a crucial engineering decision, governed by the required cooling rate for a specific steel, as well as practical considerations such as cost, availability, and operational safety. ### 1.1 General Requirements for Quenchants An ideal quenchant should possess a combination of properties that make it effective, economical, and safe to use. The essential requirements include: - Cost: The medium should be inexpensive. - Availability: It should be readily and widely available. - Non-corrosive nature: It should not corrode the workpiece or the quenching equipment. - Non-hazardous nature: The quenchant should be non-toxic and non-flammable. - Ease of handling and disposal: It should be simple to manage during its operational life and easy to dispose of responsibly. ### 1.2 Water Water is one of the most common and fundamental quenching media. - Primary Application: It is widely used for plain carbon steels, which have low hardenability and require a very fast cooling rate to form martensite. - Quenching Power: Its cooling power is less than that of aqueous solutions like brine or caustic soda but is significantly higher than other media like oil or molten salts. - General Suitability: It fulfills all the general requirements of an ideal quenchant (low cost, high availability, non-hazardous). ### 1.3 Aqueous Media Aqueous media are water-based solutions designed to provide a faster cooling rate than plain water. Their primary function is to suppress the formation of a stable vapor film around the workpiece, which insulates it and slows down heat extraction. - Composition: Common examples include brine solutions (10 wt% NaCl / CaCl₂ in water) and caustic solutions (10 wt% NaOH in water). - Relative Quenching Power: These solutions offer the highest quenching power, with a general ranking of: Caustic > Brine > Water. - Mechanism: The addition of salts alters the physical properties of the water, including its conductivity, viscosity, and boiling point. During the initial stage of quenching (Stage I), salt crystals deposit on the hot surface of the steel. These crystals then violently vaporize and explode, which disrupts and breaks the insulating vapor film. This forces the workpiece to enter the more efficient boiling phase of cooling (Stage II) almost immediately. - Primary Drawback: The principal disadvantage of these media is their chemical aggressiveness. Brine (NaCl) is corrosive but non-hazardous, whereas caustic solutions are both corrosive and hazardous, requiring careful handling and specialized equipment. ### 1.4 Molten Salts Molten salts are used for specialized quenching applications where precise temperature control and slower, more uniform cooling rates are necessary. - Primary Use: Their main application is for interrupted quenching processes, such as martempering and austempering, particularly for high-hardenability tool steels. This involves quenching the steel to a specific temperature above the martensite start (Ms) temperature, holding it to equalize the temperature throughout the cross-section, and then cooling it further. - Key Advantage: A liquid salt bath provides a highly uniform temperature. This is crucial because it minimizes thermal gradients across the component's cross-section, which in turn prevents the differential cooling rates and transformation times that are the primary cause of warping and quench cracking. - Composition and Operating Temperatures: Different salt compositions are used to achieve specific operating temperature ranges. | Composition | Melting Point (°C) | Operating Temp (°C) | |-------------|--------------------|---------------------| | 50% NaNO₂ + 50% KNO₂ | 143 | 160 - 500 | | 50% NaNO₃ + 50% KNO₃ | 225 | 230 - 550 | | 100% KNO₃ | 337 | 350 - 500 | | 100% NaNO₃ | 370 | 400 - 600 | | 50% NaOH + 50% KOH | 280 | 300 - 400 | | 50% BaCl₂ + 20% NaCl + 30% KCl | 540 | 570 - 900 | - Metallurgical Requirement: For molten salts to be used effectively in interrupted quenching, the steel's Continuous Cooling Transformation (CCT) curve must be shifted significantly to the right (away from the y-axis). This provides a large enough time window to cool the part to the salt bath temperature without an undesirable transformation to pearlite or ferrite. The careful selection of these cooling media is foundational to heat treatment, but properties can be further enhanced by integrating mechanical work with thermal cycles, a strategy known as thermo-mechanical treatment. ## 2.0 Thermo-Mechanical Treatments (TMT) Thermo-Mechanical Treatment (TMT) is an advanced metallurgical process that strategically combines plastic deformation with heat treatment. This synergy allows for the development of mechanical properties—particularly strength and toughness—that are superior to those achievable through conventional heat treatment alone. TMT is a critical pathway for strengthening low-carbon steels (e.g., < 0.3% C) that cannot be effectively hardened by traditional quench-and-temper methods because they are essentially non-hardenable. ### 2.1 Fundamental Principles The core principle of TMT is the intentional introduction of a high density of crystalline defects—such as dislocations, vacancies, and stacking faults—into the austenitic matrix via plastic deformation. These defects then serve as potent, additional nucleation sites during the subsequent cooling transformation. This abundance of nucleation sites leads to a profound refinement of the final microstructure (e.g., finer ferrite grains or bainite laths), which is the primary source of the enhanced mechanical properties. ### 2.2 HTMT vs. LTMT TMT processes are broadly classified into two categories based on the temperature at which deformation occurs relative to the austenite's recrystallization temperature. | Feature | High-Temperature TMT (HTMT) | Low-Temperature TMT (LTMT) | |---------|----------------------------|---------------------------| | Process Description | Plastic deformation of austenite is carried out above its recrystallization temperature. | Austenite is supercooled to a temperature below its recrystallization temperature before being plastically deformed. | | Relative Strength | Lower strength compared to LTMT. | Higher strength due to significant work hardening. | | Relative Ductility | Higher ductility compared to LTMT. | Lower ductility due to the extensive work hardening effect. | ### 2.3 Key TMT Processes Several specific TMT processes have been developed to target different microstructures and applications. #### 2.3.1 Controlled Rolling Controlled Rolling is an HTMT process applied primarily to microalloyed steels, also known as High-Strength Low-Alloy (HSLA) steels. - Process Steps: The steel is heated to a temperature above the A₃ transformation point but kept relatively low (below 1050°C) to prevent excessive grain growth. It then undergoes plastic deformation via rolling. Crucially, quenching is avoided as no martensite formation is desired; instead, the steel is air-cooled, which introduces some thermal stresses but achieves the target microstructure. - Final Microstructure: The process yields a very fine-grained ferrite + fine pearlite structure, which provides an excellent combination of strength and toughness. - Role of Alloying Elements: HSLA steels contain small amounts of strong carbide-forming elements like Niobium (Nb), Titanium (Ti), and Vanadium (V). These elements dissolve during the initial heating. Upon cooling and during deformation, they precipitate as very fine carbides, which pin austenite grain boundaries and inhibit grain growth, leading to the final refined microstructure. - Applications: The resulting high strength-to-weight ratio makes these steels ideal for demanding applications, including space exploration, deep-sea structures, and oil & gas pipelines. #### 2.3.2 Ausforming Ausforming is a classic LTMT process designed to produce exceptionally high strength in alloy steels. - Process Steps: 1. The steel is heated above A₃ to form a fully austenitic structure. 2. It is rapidly supercooled to a temperature below the recrystallization temperature but still within the metastable austenite region of the TTT diagram. 3. The steel is heavily deformed (e.g., by rolling or forging) while in this austenitic condition. 4. Finally, it is water quenched to transform the heavily deformed austenite into martensite and/or bainite. - Ideal Materials: This process is most effective for steels whose TTT diagrams are shifted to the right, providing a wide "bay" of metastable austenite. Alloying elements like Molybdenum (Mo) are particularly useful as they separate the pearlite and bainite transformation regions, offering more time for deformation. - Final Microstructure: The final structure is predominantly Bainite + Martensite, often with some retained austenite. It is characterized by very fine plates and an extremely high dislocation density inherited from the deformed austenite. - Property Comparison: The benefits of ausforming are significant, as shown in the table below for a steel with C: 0.35%, Ni: 4.5%, Cr: 1.5%. | Property | Conventional H&T | Ausforming | |----------|------------------|-----------| | Y.S. (MPa) | 2035 | 2760 | | HRC | 56 | 58 | | %e | 2 | 12 | | %RA | 5 | 42 | - Key Process Variables: 1. Austenitizing temperature: Determines the initial austenite grain size. 2. Cooling rate: Must be fast enough to avoid pearlite formation. 3. Deformation temperature: Must be below the recrystallization temperature. 4. Deformation rate: Affects the degree of work hardening. 5. Alloying elements: Control the TTT diagram shape and stability of austenite. #### 2.3.3 Isoforming & Thermomechanical Annealing - Isoforming: In this process, the steel is supercooled to the pearlite (P) or bainite (B) transformation region and is then heavily deformed during the austenite-to-pearlite (γ → P) or austenite-to-bainite (γ → B) transformation. The final cooling rate is not a critical parameter. - Thermomechanical Annealing: This process involves deforming the austenite during a very slow cooling process (annealing). This simultaneous deformation and transformation produces a very fine pearlite structure. While TMT modifies the bulk properties of a component, many applications require properties that are specific to the surface, leading to another class of heat treatments. ## 3.0 Surface Hardening Techniques Surface hardening, or case hardening, is a set of processes used to create a component with a dual microstructure: a hard, wear-resistant surface (the "case") and a tough, ductile interior (the "core"). This combination is ideal for components subjected to high surface loads and impacts, such as gears, axles, and pulleys. ### 3.1 Two Core Categories Surface hardening techniques can be divided into two main categories based on how the surface properties are altered. 1. Category I: The surface layer is converted to martensite without changing its chemical composition. This approach is applicable only to steels with sufficient carbon content (a minimum of 0.35 wt% C) to form hard martensite upon quenching. 2. Category II: The chemical composition of the surface layer is changed by diffusing elements like carbon and/or nitrogen into it. This is followed by heat treatment to harden the enriched case. This method is used for low-carbon steels to create a hard case while maintaining a tough, ductile core. ### 3.2 Category I: Hardening without Compositional Change These methods rely on selective heating of the surface followed by rapid cooling. #### 3.2.1 Flame Hardening This process uses a high-temperature oxy-acetylene flame to rapidly heat the surface of the steel into the austenitic range, followed by quenching. The four common methods are: - Spot/Stationary: Both the torch and the workpiece are stationary. - Progressive: The torch moves along the surface of a stationary workpiece. - Spinning: The workpiece spins while the torch is held stationary. - Progressive Spinning: The workpiece spins while the torch moves along its length. Advantages: - There is no restriction on the size or shape of the component. - It allows for localized hardening of specific areas. - The core microstructure and properties remain unaltered. Key Limitations: - The steel must have a minimum carbon content of 0.35%. - It is very difficult to precisely control the surface temperature and the resulting case depth. #### 3.2.2 Induction Hardening This technique uses electromagnetic induction to heat the steel surface. An alternating current (AC) is passed through a copper coil, generating a magnetic field that induces eddy currents in the component. The electrical resistance of the steel causes rapid, localized heating (the "skin effect"). - Case Depth Calculation: The total hardened depth is a function of the electrical properties of the steel, the AC frequency, and conduction time. - Depth of penetration of eddy currents: d=5000 √(μf/ρ) (mm) - Additional depth due to conduction: d_add = 0.2 √t (mm) - Total depth: d_total = (5000 √(μf/ρ)) + 0.2 √t (mm) - Where: ρ is electrical resistivity, μ is magnetic permeability, f is frequency, and t is time in seconds. - Ideal Prior Microstructures: For effective hardening, the prior microstructure should be easily austenitized. Preferred structures include hardened and tempered martensite or normalized structures with fine pearlite. A structure with large spheroidal carbides is less desirable; the rapid heating cycle of induction hardening may not provide sufficient time for these stable carbides to fully dissolve into the austenite, leading to incomplete hardening. Advantages: - Allows for precise control of case depth and temperature. - It is a very rapid process. - Produces a fine-grained martensitic case. Limitations: - Requires a minimum of 0.35% C. - Difficult to apply to complex shapes due to the need for custom coils. - Involves a high initial investment cost for equipment. #### 3.2.3 Laser Beam Hardening In this method, a high-power laser beam is used to rapidly heat a localized area on the steel's surface. A key feature of this process is its reliance on self-quenching. - Mechanism: The large, cool mass of the component acts as an efficient heat sink, drawing heat away from the thin, heated surface layer so rapidly that it quenches to form martensite without an external quenchant. - Process Parameters: Case depth is controlled by energy input over time. A lower travel speed increases the laser's dwell time on any given point, allowing heat to conduct deeper into the material. Similarly, a lower power density (while still sufficient for austenitization) reduces the risk of surface melting, enabling longer interaction times for deeper heating, ultimately resulting in a larger case depth. Advantages: - Extremely precise control over the hardened area and depth. - The beam can reach remote or inaccessible locations using optics. - Self-quenching eliminates the need for a separate quenching step under favorable conditions. Limitations: - Very high initial investment cost. - There is a danger of melting the surface if parameters are not controlled. - Steel has poor laser absorption, often requiring surface coatings (e.g., manganese phosphate, graphite) to improve efficiency. #### 3.2.4 Electron Beam Hardening This process is conceptually similar to laser hardening but uses a focused beam of high-energy electrons to heat the surface. It is capable of hardening microscopic areas with sub-micron resolution. Its primary operational requirement is the need for a vacuum environment to prevent the electrons from scattering. ### 3.3 Category II: Hardening with Compositional Change These thermochemical processes enrich the surface of low-carbon steels with interstitial elements to make the case hardenable. For any thermochemical surface treatment to be successful, three sequential kinetic steps must occur: 1. Liberation: Generation of the diffusing species (e.g., carbon or nitrogen) in an atomic state from a solid, liquid, or gas source. 2. Absorption: The atomic species is absorbed onto the surface of the steel. 3. Diffusion: The absorbed atoms move from the high-concentration surface into the bulk material, creating a concentration gradient. #### 3.3.1 Carburizing Carburizing introduces carbon into the surface of a low-carbon steel. The process is always performed at a high temperature (typically 900-950°C) where the steel is in the austenite (γ) phase. This is because carbon has a much higher solubility in FCC γ-iron than in BCC α-iron, allowing for efficient diffusion. - Pack Carburizing (Solid): The steel component is packed in a sealed box with a solid carburizing mixture. This mixture typically consists of hardwood charcoal (85%, as the carbon source), Barium Carbonate (BaCO₃, 12%, as an energizer to promote CO gas formation), and Calcium Carbonate (CaCO₃, 3%), which prevents the caking of the charcoal. The box is heated for several hours. This method is the cheapest but offers poor control over uniformity and has high labor costs. - Liquid Carburizing: The component is immersed in a molten cyanide salt bath at 900-950°C. This process is faster but involves highly toxic cyanide compounds, making handling and disposal challenging. - Gas Carburizing: The component is heated in a gas-tight furnace containing a controlled atmosphere. A neutral carrier gas (a mixture of N₂, H₂, and CO) is introduced along with a small amount of a carburizing gas, such as propane (C₃H₈) or methane (CH₄). This method provides the best control over case depth and carbon concentration but requires the highest equipment investment. #### 3.3.2 Nitriding Nitriding is a process that diffuses nitrogen into the surface of steel to create a very hard case. - Process: It is performed at a lower temperature range (480-575°C), where the steel is in the ferritic condition. The nitrogen source is typically anhydrous ammonia (NH₃) gas, which decomposes at the steel's surface to liberate atomic nitrogen. - Hardening Mechanism: Nitriding is most effective for alloy steels that contain strong nitride-forming elements like Aluminum (Al), Chromium (Cr), and Molybdenum (Mo). The hardness is not due to martensite formation but from the precipitation of extremely fine (10-15 nm), hard, and uniformly distributed alloy nitride particles. The formation of these potent strengthening precipitates is governed by the slow diffusion of the substitutional alloying elements (Al, Cr, Mo), not the rapid diffusion of interstitial nitrogen. This kinetic limitation is what keeps the particles extremely fine and effective at blocking dislocation motion. #### 3.3.3 Carbonitriding Carbonitriding is a modified carburizing process where both carbon and nitrogen are diffused into the steel surface simultaneously. This can be done in a liquid medium using cyanide baths or in a gas atmosphere by adding both a carbon source (e.g., CH₄) and a nitrogen source (NH₃) to the carrier gas. The success of all these heat treatment strategies—from quenching to TMT to surface hardening—is ultimately dictated by the steel's chemical composition, particularly the presence of alloying elements. ## 4.0 The Role of Alloying Elements in Steel Plain carbon steel, an alloy of iron and carbon, has several inherent limitations that restrict its performance in many applications. Alloying elements are intentionally added to enhance properties and overcome these drawbacks, which include: - Very low hardenability, limiting hardening to thin sections. - Significant loss of hardness upon tempering. - Poor resistance to oxidation and corrosion. ### 4.1 AISI Steel Designation System The American Iron and Steel Institute (AISI) established a numerical system to classify steels based on their chemical composition. The most common format is a 4-digit number (though 5-digit numbers are also used). - 1st digit: Indicates the principal alloying element type (e.g., 1 for carbon steels, 2 for nickel steels, 4 for molybdenum steels). - 2nd digit: Represents the approximate percentage of the major alloying element. - 3rd & 4th digits: Together, these indicate the carbon content in hundredths of a percent. For example, '40' signifies a carbon content of 0.40 wt% C. Examples: - AISI 1040: A plain carbon steel (10xx series) with approximately 0.40% carbon. - AISI 2560: A nickel steel (2xxx series) with approximately 5% nickel and 0.60% carbon. ### 4.2 Classification of Alloy Steels Alloy steels can be classified in several ways depending on the criteria of interest: - Based on Alloy Content: - Low-alloy steel: Total alloy content is less than 5%. - Medium-alloy steel: Total alloy content is between 5% and 10%. - High-alloy steel: Total alloy content is greater than 10%. - Based on Composition (Major Alloying Element): - Ni-steels, Cr-steels, Mo-steels, etc. - Based on Structure: - Ferritic, Austenitic, Martensitic, and Duplex (mixed-phase) steels. - Based on Application: - Tool steels, Transformer steels, Heat resistant steels. ### 4.3 Influence of Alloying Elements on Phase Stability Alloying elements have a profound effect on the stability of the different phases of iron (ferrite and austenite). - Ferrite Stabilizers: Elements like Chromium (Cr), Tungsten (W), Molybdenum (Mo), and Vanadium (V) have a BCC crystal structure, while Silicon (Si) has a diamond cubic structure. These elements stabilize the BCC ferrite phase. At high enough concentrations, they can create a "gamma loop" in the phase diagram, which shrinks and can completely eliminate the austenite phase region. - Austenite Stabilizers (FCC Structure): Elements like Manganese (Mn), Nickel (Ni), Cobalt (Co), and Copper (Cu) have an FCC structure, similar to austenite. They expand the temperature and composition range over which austenite is stable, effectively lowering the temperature at which austenite transforms to ferrite. ### 4.4 Influence on Eutectoid Point and TTT Diagram Alloying elements alter the kinetics and thermodynamics of the eutectoid transformation, which is fundamental to heat treatment. - Effect on Eutectoid Composition: Most alloying elements shift the eutectoid point to a lower carbon content. - Effect on Eutectoid Temperature: The effect varies by element. Some elements like Titanium (Ti), Molybdenum (Mo), and Silicon (Si) raise the eutectoid temperature, while others like Nickel (Ni) and Manganese (Mn) lower it. - Effect on TTT Diagram: With the exception of Cobalt (Co), nearly all alloying elements shift the transformation curves (pearlite and bainite noses) to the right, meaning longer times are required for these transformations to begin. This 'shift to the right' is fundamentally what increases the steel's hardenability; it provides a larger time window for the workpiece to cool past the 'nose' of the pearlite curve, thus avoiding soft transformation products and allowing martensite to form at slower cooling rates or in thicker sections. Certain elements, notably Molybdenum (Mo), can introduce separate "bays" for the pearlite and bainite transformations. An Introduction to Surface Hardening of Steel 1. The Core Idea: A Tough Heart with a Hard Shell Imagine a critical machine part, like a gear in a car's transmission. It needs to be incredibly hard on the outside to resist constant grinding and wear, but it also needs to be tough and flexible on the inside to absorb shocks without shattering. If the entire gear were extremely hard, it would be too brittle; if it were entirely tough, it would wear out quickly. This is the fundamental purpose of surface hardening: to create a dual-property component with a hard, wear-resistant outer layer (the "case") while maintaining a tough, ductile inner section (the "core"). This unique combination is essential for the performance and longevity of engineering components like gears, axles, and pulleys. For some parts, like drill bits, this hardening is highly localized—only the tip of the surface is hardened to provide a cutting edge while the shank remains tough. The primary goals of surface hardening are: • To improve wear resistance. • To improve fatigue resistance. • To improve corrosion resistance. To achieve this "hard shell, tough heart" structure, metallurgists use two fundamental approaches. 2. Two Paths to a Harder Surface: The Main Categories Surface hardening techniques are broadly divided into two main categories based on how they treat the steel's surface. 1. Category I: Processes that harden the surface using heat treatment without changing its chemical composition. 2. Category II: Processes that harden the surface by first changing its chemical composition, followed by a heat treatment. Let's explore the methods within each of these categories. 3. Category I: Hardening with Heat Alone Unlike the methods we'll see later, these processes do not add new elements to the steel; they work by transforming the carbon already present in the surface into a hard microstructure. The core principle is to rapidly heat and quench only a thin surface layer. This is only possible for steels that already have enough carbon (a minimum of 0.35% C) to be hardened. The rapid heating and cooling cycle transforms the surface into an extremely hard microstructure called martensite, leaving the tough inner core completely unaltered. 3.1. Flame Hardening This process uses a high-temperature oxy-acetylene flame to quickly heat the surface of the steel before it is quenched. This process can be applied in several ways, such as stationary spot hardening or progressively scanning the flame along a component's length or circumference (known as progressive, spinning, or progressive spinning methods). Advantages Limitations No restriction on component size or shape. Difficult to precisely measure temperature. Allows for localized hardening. Precise control of case depth is difficult. The structure of the core remains unaltered. 3.2. Induction Hardening Induction hardening uses an alternating current (AC) passed through a copper coil. This generates a magnetic field that, in turn, creates powerful eddy currents on the surface of the steel part placed within the coil. The steel's natural resistance to these eddy currents generates intense, instantaneous heat. This phenomenon, known as the "skin effect," concentrates the heating only on the surface, which is then quenched. Advantages Limitations Core properties are unaltered. Difficult to design coils for complex shapes. Offers precise control of case depth. High initial investment cost. Creates a uniform hardened case. Produces finer martensite. A rapid process suitable for automation. Cheaper for mass production. 3.3. High-Energy Beam Hardening (Laser & Electron Beam) These advanced methods use a focused beam of high energy—either from a laser or an electron beam—to heat a very specific, localized area on the steel's surface. • A key advantage of laser hardening is its ability to harden remote or difficult-to-reach areas by using optics to direct the beam. It also offers precise control over case depth. • However, laser hardening has practical limitations, including the danger of melting the surface if not properly controlled. Furthermore, steel's surface is reflective, so an absorptive coating (like manganese phosphate or graphite) is often required. • A unique feature of these methods is "self-quenching." If the overall component is large enough, the bulk of the cold metal acts as a heat sink, drawing heat away from the surface so rapidly that it quenches itself without needing an external liquid quenchant. • Electron beam hardening is very similar to laser hardening but has one critical requirement: the process must be performed in a vacuum. These methods are highly effective for steels with sufficient inherent carbon. However, for low-carbon steels that prioritize a tough core, a different approach is required—one that modifies the surface chemistry itself. 4. Category II: Changing the Surface Chemistry These methods are designed for steels that don't have enough carbon to be hardened by heat alone (e.g., low-carbon steels with < 0.3% C). The goal is to first enrich the surface of the steel by diffusing new elements into it—most commonly carbon or nitrogen. This enrichment creates a new, high-carbon or high-nitrogen "case" that can then be hardened by heat treatment. 4.1. Carburizing Carburizing involves heating a low-carbon steel component in a high-temperature (900-950°C), carbon-rich environment. At this temperature, carbon atoms from the environment diffuse into the surface of the steel, creating a high-carbon case. There are three main ways to perform carburizing, based on the carbon source: • Pack Carburizing: The component is packed into a sealed box filled with a solid, powdered mixture, such as hardwood charcoal. A practical challenge here is that the solid mixture is a poor heat conductor, meaning the entire box cannot be quenched directly. The component must be removed from the hot box, causing it to air cool. A second, separate heat treatment is then required to reheat the part for a short time and quench it to achieve the final hardness. • Liquid Carburizing: The component is submerged in a molten bath of cyanide salts. • Gas Carburizing: The component is heated in a sealed furnace filled with a carbon-rich gas, such as propane or methane. 4.2. Nitriding Nitriding is a process where a steel part is heated to a relatively low temperature (480-575°C) in the presence of ammonia gas (NH₃). The ammonia breaks down, and nitrogen atoms diffuse into the surface. The key insight for nitriding is that it is most effective for alloy steels containing elements like aluminum (Al), chromium (Cr), or molybdenum (Mo). The diffused nitrogen doesn't just enrich the iron; it reacts with these alloying elements to form a layer of extremely hard, fine nitride particles. This hardening mechanism is different from carburizing, which relies on creating martensite, and is so effective that it can produce surface hardness values up to Rc 75. 5. Summary: Choosing the Right Path The choice between hardening with heat alone or by changing the surface chemistry depends entirely on the starting material and the desired outcome. The table below provides a simple comparison. Feature Category I (e.g., Flame, Induction) Category II (e.g., Carburizing, Nitriding) Core Principle Converts existing carbon on the surface into a hard structure (martensite) using heat. Adds new elements (carbon or nitrogen) to the surface, then heat treats. Suitable Steel Type Medium- to high-carbon steels (typically > 0.35% C). Low-carbon steels (typically < 0.3% C) that require hardening. Primary Goal Harden the existing surface. Create a new, hardenable surface composition.

HT preQuiz2