The Heat-Affected Zone (HAZ) is one of the most critical aspects of welding metallurgy. It's the area of base metal that is not melted but has undergone significant changes in its microstructure due to exposure to high temperatures during welding. The HAZ can affect the mechanical properties of the metal, such as its hardness, toughness, and susceptibility to cracking. Controlling the HAZ is crucial in maintaining the integrity of the weld joint and the overall structure.

1. What is the Heat-Affected Zone (HAZ)?

The HAZ refers to the portion of the base material adjacent to the weld that has experienced thermal cycles (heating and cooling) intense enough to alter its microstructure, but not enough to melt it. While the weld pool itself forms the fusion zone (FZ), the HAZ surrounds this area and is divided into various temperature gradients, each affecting the material differently.

In many materials, especially carbon steels, stainless steels, and alloy steels, the HAZ is a critical factor in weld performance. The thermal history that the HAZ experiences during welding can induce hardness, brittleness, grain growth, and potential cracking if not carefully managed.

2. Metallurgical Changes in the HAZ

The changes that occur in the HAZ depend on several factors, including the material composition, the welding process, and the cooling rate. The HAZ can be broken down into three key subzones:

  • Coarse Grain Heat-Affected Zone (CGHAZ): Closest to the fusion zone, the CGHAZ experiences the highest temperatures just below the melting point of the base material. In steel, this causes grain growth and significant microstructural changes. Coarser grains result in reduced toughness, making the material more susceptible to cracking.

  • Fine Grain Heat-Affected Zone (FGHAZ): As you move away from the fusion zone, the metal experiences lower temperatures, leading to finer grain structures. Finer grains improve toughness and ductility compared to the coarse-grain zone.

  • Intercritical and Subcritical HAZ: These regions are farthest from the fusion zone and experience temperatures below the transformation point. The subcritical HAZ undergoes tempering, while the intercritical zone sees partial phase transformations. In steels, this area might include a mix of ferrite and pearlite or other phases, depending on the material.

In materials like aluminum alloys, the HAZ can cause precipitate dissolution and over-aging, reducing the material’s strength, which can be problematic in aerospace applications.

3. Effect of Welding Parameters on the HAZ

The extent and properties of the HAZ are highly dependent on the welding process parameters:

  • Heat Input: This is a critical factor influencing the size and properties of the HAZ. Heat input is determined by the welding process, current, voltage, and travel speed. A high heat input increases the size of the HAZ and can lead to grain coarsening and softening of the base metal in steels, increasing the risk of cracking.

    Formula: Heat Input (kJ/mm) = (Voltage * Current * 60) / (1000 * Travel Speed)

  • Cooling Rate: The cooling rate after welding has a significant impact on the microstructural evolution of the HAZ. Rapid cooling in steels can lead to the formation of martensite, a hard but brittle phase, making the weld joint more prone to cracking. Controlled cooling, such as post-weld heat treatment (PWHT), can relieve residual stresses and temper martensitic structures, enhancing toughness.

  • Welding Technique: The use of multi-pass welding (especially in thicker materials) can alter the thermal cycles experienced by the HAZ, with subsequent passes reheating and tempering previously welded areas. This can improve the toughness of the HAZ.

4. Common Problems Associated with the HAZ

  • HAZ Cracking: Cracking in the HAZ is a common issue, especially in high-strength steels or thick sections. Hydrogen-induced cracking (HIC) or cold cracking often occurs due to the combination of a high hardness HAZ, residual stresses, and hydrogen absorption during welding.

  • Brittleness and Hardness: If the HAZ experiences too much grain coarsening or forms martensitic structures in steels, it can become excessively hard and brittle, increasing the risk of brittle fracture under stress.

  • Softening in Aluminum: In heat-treated aluminum alloys, such as 6061, the HAZ can experience precipitate dissolution, leading to softening. The strength of the aluminum alloy is significantly reduced in the HAZ compared to the parent material.

5. Controlling the HAZ

To ensure optimal weld performance and minimize problems in the HAZ, several control methods are used:

  • Preheating: Preheating the base material before welding helps reduce the cooling rate, minimizing the risk of HAZ hardening and cracking, especially in carbon steels. Preheating temperatures depend on the material but can range from 150°C to 300°C.

  • Post-Weld Heat Treatment (PWHT): PWHT is a thermal process applied after welding to relieve residual stresses and improve toughness in the HAZ. In steels, PWHT reduces the hardness of martensite and improves ductility. The process typically involves heating the welded assembly to a temperature just below the transformation range and holding it for a specified time.

  • Low-Hydrogen Electrodes: Using low-hydrogen electrodes (such as E7018 for stick welding) or properly controlled shielding gases reduces hydrogen content in the weld, minimizing the risk of hydrogen-induced cracking in the HAZ.

  • Optimizing Heat Input: By using controlled heat input processes, such as pulsed MIG or TIG welding, welders can reduce the size of the HAZ and minimize grain growth. Pulsed techniques deliver high energy only during certain parts of the welding cycle, which controls the amount of heat absorbed by the base material.

6. Modern Techniques to Minimize HAZ Damage

Recent advancements in welding technology offer new ways to reduce the impact of the HAZ:

  • Laser Welding: Laser welding provides a highly focused heat source, minimizing heat input and significantly reducing the size of the HAZ. This technique is ideal for materials like stainless steel and titanium.

  • Electron Beam Welding: Like laser welding, electron beam welding delivers high energy density, reducing the HAZ and associated metallurgical changes.

Conclusion

The Heat-Affected Zone is a complex but critical aspect of welding that can significantly impact the performance of welded joints. Understanding how metallurgical changes in the HAZ occur and how to control them through process parameters, preheating, and post-weld treatments is essential for achieving strong, reliable welds. Proper control of the HAZ ensures longevity, reduces cracking risks, and optimizes the mechanical properties of the welded joint.

For more insights on welding techniques and advanced equipment, contact Quantum Machinery Group at Sales@WeldingTablesAndFixtures.com or call (704) 703-9400.

High Speed Door Accessories

About high speed Door Accessories:

1.Driving system: High Speed Door Servo System motor and control, motor SNMA / SEJ +reducer+flang+encoder+control box

2.Other accessories: Middle aluminium bar, bottom aluminium bar, aluminium vertical track, stainless steel vertical track, brushes,round nylon strip, PVC roll, rolling steel tube complete, drum be;;s for rolling tube, side end covers, stainless steel cover, steel tubes, air bag sensor system, safety light beam, Microwave sensor 24 Gigahertz,PVC welding machine, smoke and fire sensors, PVC transparent window, magnetic loop, photocell, remote control, manual push button, sound warning alarm Led lamp traffic light

For some main accessories, here is detain introduction:

1).24G radar sensor:

Brand: Hofic

Technology: Microwave processor

Maintain time: 0.5s

Protection: IP54

Transmitting frequency: 24.125GHz

Maximum installation height: <5meter

Transmitting power density: <5mw/cm2

Power consumption: <1W(VA)

Installation angle: 0-90°(lengthways) -30-30°(crosswise)

Supply voltage: AC&DC 12V-30V

2).Photocell:

Approval CE & ISO9001

Power supply AC/DC 12-30V

Static current 25A

Action current 40mA

Mounting distance Max 10m

NO or NC contact Done by NO./NC. contact selection

Working environment temperature -42℃ to +45℃

Working environment humidity 10 to 90%RH

Appearance dimensions( controller) 123*50*32mm(L*W*H)

Appearance dimensions(Magic eye head) 19*13mm(L*D)

3). Remote control

Operating Voltage: AC220V/50HZ+-20%

Transmit power:≤100mW;Super regenerative module, strong anti-interference

Operating temperature:-30℃-+70℃

Working frequency: 433MHZ;transmitter 1527, millions of encoding chip

Accept sensitivity:≤110dB

Manipulation method: electric remote control

Configure the quantity: Wireless number; factory configuration transmitter two. Can be equipped with PT2272 digital decoder chip

Configure the function: Up, down, stop, anti-lock full-featured reproduction

4). Magnetic loop

Voltage: 12V AC/DC +-10% 24V 115V AC+-10% 230V

Power: <2VA

Continued electrical output: 7A / 250V AC

Frequency range: 20 to 200K Hz

Response time: infinity (factory setting) or adjust maintain time

Sensitivity: 0.01% to 0.1% divided into 3 levels

Protection level: IP30

Electromagnetic compatibility (EMC) Complies with EM50081-1 and EN50082-2

Detection loop inductance: the limit of 40uH to 100uH ideal value 80uH to 300uH

Operating temperature: -25 ℃ to 70 ℃

Storage temperature: -40 ℃ to 85 ℃

Relative humidity: <90% non-condensing

Temperature compensation: automatic compensation up to 50 ℃ / h

Connection type: 11 pin DIN rail socket

Dimensions: 41 * 78 * 85 mm (width * height * length)

Weight: 250 grams

5). manual push button

This exit switch button is made of ABS and has built-in indicator light make it looks more elegant and fashionbale. It can be widely used in public entrance gates, public door, home, office, hotel, etc.

Door Exit Button Push To Exit Switch Door Opener For Access Control Security

Main Feature

Fashionable appearance

Made of ABS Plastic

Easy to install and operate

Used for access control

Specification

Mechanical Life: 500,000 test

Material: Plastic

6). Limit switch encoder

Voltage: DC5V-24V

Output voltage: high voltage≥85%Vcc, low voltage≤0.5V

Maximum mechanical speed:6000rpm

Anti-vibration: 50m/s,10-200HZ,XYZeach way 2 times.

Current consumption:≤30mA

Response frequency: 0-100KHZ

Protection: waterproof,oil proof,dust prevention, IP54

The output code system: binary code

Operating life: MTBF≥30000(+25℃,2000rpm)

Rotational inertia:4.0x10-8kgm²

Maximum load:radial direction20N

Axial direction:10N

Weight: about 0.4kg

,Electronic High Speed Door Accessories

Shenzhen Hongfa Automatic Door Co., Ltd. , https://www.hongfadoor.com