What Is Heat Treatment? Procedure and Standards

Heat treatment is a crucial process used in metallurgy to modify the properties of materials, particularly metals, and alloys. It involves controlled heating and cooling to achieve specific outcomes like increased hardness and improved strength. In this article, we will explore the different methods and applications of heat treatment and its significant impact on shaping material properties.

What Is Heat Treatment?

Heat treatment (or heat treating) encompasses a set of industrial, thermal, and metalworking procedures employed to modify the physical and chemical characteristics of a material. Heat treatment uses heating or cooling methods to extreme temperatures to achieve the desired results such as material hardening or softening.

By subjecting the material to controlled heating or cooling, heat treatment alters its physical and chemical characteristics, making it stronger, more durable, and resistant to wear and tear.

This treatment method is essential for ensuring the reliability and safety of critical components used in equipment and machinery within the oil and gas sector. Through heat treatment, metals can be optimized to perform their best under challenging operating conditions. 

How Does Heat Treatment Work?

Heat treatment involves three crucial stages: gradual heating for uniform temperature, soaking at a specific temperature, and cooling to room temperature. This process optimizes metal properties for enhanced strength and reliability, vital in industries like oil and gas. Mastering these stages ensures top performance in machinery and equipment.

1. Heating Stage

During the initial heating stage, the primary objective is to ensure uniform heating of the metal. Achieving even heating is accomplished by employing a gradual heating process. Uneven heating can lead to uneven expansion, resulting in distorted or cracked sections of the metal. The rate of heating is determined by several important factors:

• Heat conductivity of the metal: Metals with higher heat conductivity heat up more quickly than those with lower conductivity.
• Metal condition: Previously hardened or stressed tools and parts should be heated at a slower pace compared to those that haven’t undergone such treatments.
• Size and cross-section of the metal: Larger or unevenly shaped parts necessitate slower heating to allow for consistent temperatures throughout the metal, reducing the risk of cracking or excessive warping.

By considering these factors, proper control over the heating rate ensures the metal’s integrity and minimizes the risk of defects during the heat treatment process.

2. Soaking Stage

The soaking stage serves the purpose of maintaining the metal at the appropriate temperature until the desired internal structure is achieved. The duration of the “soaking period” is determined based on the metal’s chemical analysis and mass. In the case of uneven cross-sections, the soaking period can be calculated based on the largest section.

It is generally advised not to abruptly raise the metal’s temperature from room temperature to soaking temperature. Instead, a gradual heating approach is employed, bringing the metal just below the temperature where the structure will change, and then holding it until the temperature becomes consistent throughout the metal.

This “preheating” step is followed by more rapid heating to reach the final required temperature. For parts with intricate designs, multiple layers of preheating may be necessary to prevent warping. By adhering to these practices, the soaking stage ensures the metal achieves the desired internal structure.

3. Cooling Stage

In the cooling stage, the process of returning the metal to room temperature varies based on the type of metal being treated. Different cooling mediums, such as gases, liquids, solids, or combinations thereof, may be required. The rate of cooling is determined by the specific metal and the chosen cooling medium. These decisions play a crucial role in achieving the desired properties of the metal.

Quenching is a rapid cooling method achieved using mediums like air, oil, water, brine, or others. It is commonly associated with hardening, as most hardened metals undergo rapid cooling through quenching. However, it is essential to note that not all rapid cooling processes result in hardening. For instance, water quenching is used to anneal copper, and some metals are hardened through slow cooling.

It is essential to exercise caution with quenching, as it can potentially crack or warp certain metals. Generally, brine or water can rapidly cool metal, while oil mixtures are more suitable for slower cooling. Following general guidelines, you can use water for hardening carbon steels, oil for hardening alloy steels, and water for quenching nonferrous metals.

Nonetheless, it is crucial to tailor the cooling rate and medium to the specific metal for optimal results in the heat treatment process.

What Are The Types of Heat Treatment?

In general, there are several types of heat treatment that are commonly employed.

1. Annealing

Annealing is a heat treatment method that alters the physical and occasionally the chemical characteristics of a material to enhance its ductility and reduce its hardness, thereby making it more workable for further processing.

In ferrous alloys, the process of annealing typically involves heating the metal above its upper critical temperature and then cooling it at a very slow rate, leading to the formation of pearlite. For pure metals and certain alloys that cannot undergo heat treatment, annealing is utilized to eliminate the hardness resulting from cold working.

2. Normalizing

Normalizing is a metal treatment process where the metal is heated below its melting point and then cooled in air to make it more ductile. During the normalizing process, the steel is heated to approximately 40 degrees Celsius above its upper critical temperature limit. It is then held at this temperature for a certain period before being cooled in the surrounding air.

3. Stress Relieving

Stress relieving is a method employed to remove residual stresses accumulated during previous manufacturing processes. This technique involves heating to a temperature below Ac1 (applicable to ferritic steels) and maintaining it at that temperature for the required time, to achieve the desired reduction in residual stresses.

Stress relieving is a frequent practice applied to items such as air tanks, boilers, and other pressure vessels to eliminate the stresses generated during the welding process.

4. Aging

The aging process in heat treatment involves raising the temperature of an alloy to alter its properties. Through a sequence of heat treatments, the process accelerates changes in the properties of an alloy.

Certain metals fall under the category of precipitation-hardening metals. When these alloys are quenched, their alloying elements become trapped in solution, leading to a soft metal state. Subsequent aging of the “solutionized” metal enables the diffusion of alloying elements throughout the microstructure, forming intermetallic particles.

5. Quenching

Quenching refers to the rapid cooling of a metal object, achieved by immersing it in oil or water, from the elevated temperature at which it was formed.

Quenching is a rapid cooling process applied to metals, primarily to induce a martensite transformation. In ferrous alloys, this results in increased hardness, while non-ferrous alloys typically become softer than usual.

6. Tempering

Tempering is a heat treatment process used to enhance the toughness of iron-based alloys. Untempered martensitic steel possesses high hardness but is too brittle for practical use in most applications. To address this issue, tempering is employed.

Tempering is essential for quenched parts in most applications. It involves heating the steel below the lower critical temperature (typically ranging from 400˚F to 1105˚F or 205˚C to 595˚C, depending on the desired outcomes) to impart a certain level of toughness.

7. Selective Heat Treatment

Differential heat treatment, also known as selective or local heat treatment, is a heat-treating technique used to harden or soften specific areas of an object, resulting in a different level of hardness between these areas.

8. Cold and Cryogenic Treating

Cryogenic treatment is a process of treating objects to cryogenic temperatures (typically around -300°F / -184°C, or even as low as −190 °C (−310°F)). This process is applied to steels and other metal alloys like aluminum to remove residual stresses and enhance wear resistance.

Even though quenching steel leads to the transformation of austenite into martensite, not all of the austenite completely transforms. Some austenite crystals may remain unchanged even after quenching below the martensite finish (Mf) temperature.

9. Decarburization

Decarburization refers to the loss of carbon in the surface-adjacent zone of the material (Gunnarson, 1963). Heating steel is occasionally employed to modify its carbon content. When steel is heated in the presence of oxygen, the iron combines with the oxygen to create an iron-oxide layer, protecting decarburization.

What Are The Heating Plans of Heat Treatment According to ASME PCC-2?

ASME PCC-2 provides guidelines for heating plans used during repairs and alterations to pressure equipment. These plans specify the necessary temperature and duration of heating based on factors like material type, thickness, and the specific repair being performed.

1. Scope

• Purpose [Explain why the heat treatment (HT) is being performed]
• Summary

-Vessel Owner

– Vessel Identification

-Governing Codes and Standards

-Material Specification(S)

-Thermal Process (Direct Firing, Resistance, Induction, Combination, Other)

-HT Soak Temperature Range (Target and Tolerance)

-HT Soak Time

-Maximum Heating Rate

-Maximum Cooling Rate

2. Sketches Or Drawings

• Location(S) of All Heating Devices Relative to Readily Identifiable Points on The Vessel Such As Nozzles.
• Location(S) of All Thermocouple(S) Relative to Readily Identifiable Points on The Vessel such as Nozzles
• Location(S) and Details of Insulation, Bulkheads, Etc.

3. Soak, Gradient, And Heating Bands

• Location
• Temperature Targets and Tolerances

4. Temperature Measurement And Control

• Make and Model of Thermocouples or Other Temperature Sensing Devices
• Calibration Procedure and Most Recent Calibration Date of Thermocouples or Other Temperature Sensing Devices
• Make and Model of Controllers and Recording Devices

5. Heating Plan

• Target Heating and Cooling Rates with Tolerance For Each Section (Soak Zone, Gradient, Etc.)
• Target Hold Times with Tolerances for Each Step of The Process
• Contingency Plans
• Complete or Partial Loss of Power, Gas, etc., Which Impacts Heating Ability
• High Winds
• Plant Emergency

6. Stability Evaluation

Stability assessment includes the inspection of the structural strength and reliability of elements under heat influence. This assessment guarantees that materials and equipment utilized in the heating procedure can endure temperature variations without jeopardizing their structural soundness. 

7. Required Items Or Support Provided By Others

• Permits
• Utilities
• Lighting
• Shelter
• Plot Space

8. Documents

• Safety Plan (Site/Job Specific)
• Reference Documents
• List of Documents to be Provided to The Owner/ User after The Job

In conclusion, heat treatment is crucial in the industrial sector, influencing material properties and performance. Understanding various heat treatment processes, such as annealing, quenching, and tempering, is vital for achieving desired material characteristics.

By following proper heat treatment guidelines, companies can enhance material strength, ductility, and reliability in industries like manufacturing, aerospace, and automotive. Implementing precise techniques aligns with industry best practices, leading to superior products and a competitive edge.

Heat treatment takes a central role among the subjects explored within PetroSync‘s ASME PCC-2 training. This specialized program delves into the intricacies of heat treatment methods, ensuring you acquire a solid understanding of their significance in enhancing material properties and structural integrity across diverse industries.

PetroSync’s commitment to excellence is vividly reflected in its ASME PCC-2 training, which comprehensively covers various heat treatment techniques crucial for effective maintenance and repair. This program equips you with essential knowledge to address material challenges and ensure operational durability.

The training is thoughtfully designed to encompass a wide spectrum of pivotal topics, enabling you to develop the skills to navigate intricate scenarios and contribute substantially to successful heat treatment activities as part of repairing pressure equipment and piping.

Elevate your proficiency by enrolling in the ASME PCC-2 training with PetroSync. Acquire practical insights into pressure equipment repair and embrace the opportunity to learn the practical application of heat treatment through PetroSync’s expert guidance. Join us in mastering the art of repair of pressure equipment for optimal industrial outcomes!

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