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.
Elevator modernization (or lift modernisation) is the process of upgrading the critical parts of the elevator in order for it to be able to handle new technology, have better performance, improve safety, and even give the aesthetics an up-to-date appeal
Most elevators are built to provide about 20 years of service, as long as service intervals specified and periodic maintenance/inspections by the manufacturer are followed. As the elevator ages and equipment become increasingly difficult to find or replace, along with code changes and deteriorating ride performance, a complete overhaul of the elevator may be suggested to the building owners.
A typical modernization consists of controller equipment, electrical wiring and buttons, position indicators and direction arrows, hoist machines and motors (including door operators), and sometimes door hanger tracks. Rarely are car slings, rails, or other heavy structures changed. The cost of an elevator modernization can range greatly depending on which type of equipment is to be installed.
Modernization can greatly improve operational reliability by replacing mechanical relays and contacts with solid-state electronics. Ride quality can be improved by replacing motor-generator-based drive designs with Variable-Voltage, Variable Frequency (VVVF) drives, providing near-seamless acceleration and deceleration. Passenger safety is also improved by updating systems and equipment to conform to current codes.
Components needs to be replaced in different elevator modernization plan
| Component | Scheme of elevator modernization plan | |||
|---|---|---|---|---|
| Refurbishment work | Partial modernization | Full modernization | Full replacement | |
| Elevator car | ||||
| Sling | No | No | by building owners | Yes |
| Door operator | No | by building owners | Yes | Yes |
| Cab design | by building owners | by building owners | Yes | Yes |
| Car doors | by building owners | by building owners | Yes | Yes |
| Car fixtures | Yes | by building owners | Yes | Yes |
| Wedges connected to the governor | No | No | Yes | Yes |
| Additional phone/intercom if they don't equipped | by building owners | Yes | Yes | Yes |
| Traveling cable | No | Yes | Yes | Yes |
| Hydraulic rams | No | No | No | Yes |
| Wire ropes | No | by building owners | Yes | Yes |
| Shaft | ||||
| Counterweight | No | No | No | Yes |
| Guide rails | No | No | No | Yes |
| Buffers | No | No | Yes | Yes |
| Electrical components | No | No | No | Yes |
| Tapehead | No | Yes | Yes | Yes |
| Limit switches | No | Yes | Yes | Yes |
| Wiring | No | Yes | Yes | Yes |
| Compensations | No | No | No | Yes |
| Lobby | ||||
| Shaft door components | by building owners | by building owners | Yes | Yes |
| Shaft doors | by building owners | by building owners | by building owners | Yes |
| Hall fixtures | Yes | by building owners | Yes | Yes |
| Machine room | ||||
| Machines | No | by building owners | Yes | Yes |
| Controllers | No | Yes | Yes | Yes |
| Removal of the selector | by building owners | Yes | Yes | Yes |
| Overspeed governor | No | No | Yes | Yes |
| Additional unintended car movement protection device if they don't equipped | by building owners | by elevator safety code in countries | ||
| Backup battery or uninterruptible power supply | No | Yes | Yes | Yes |
Benefits of elevator modernization
Modernization can greatly improve operational reliability by replacing mechanical relays and contacts with solid-state electronics. Ride quality can be improved by replacing motor-generator-based drive designs with Variable-Voltage, Variable Frequency (VVVF) drives, providing near-seamless acceleration and deceleration. Passenger safety is also improved by updating systems and equipment (such as double brake system, unintended car movement protection device on the brake system etc.) to conform to current codes.
With CEP traction elevator modernization, we use the existing elevator structure while upgrading the ride quality and reliability with state-of-the-art components. Upgrade to a CEP closed loop door operator or a sophisticated, energy saving VVVF inverter.
CEP traction elevator modernization can include:
- CEP Microprocessor Based Control System
- CEP Microprocessor Based Efficient Dispatch System
- Energy Efficient AC Drives or SCR Drives on Existing AC Gearless Machines
- Traction Machines (Geared / Gearless)
- Rope Brake System
- CEP Closed Loop Door Operating System
- Car & Hall Pushbutton Fixtures & Indicators
- Car & Counterweight Roller Guides
- Seismic Upgrades
- Security Upgrades
- Cab Interior Renovations or Cab Replacement
NOTE: Complete tear-out modernizations are available.
Contact us TODAY for Right Modernization Package Available
OTIS Elevator Modernization, Thyssen Elevator Modernization, KONE Elevator Modernization, GiantKONE Elevator Modernization, ThyssenKrupp Elevator Modernization, Sch****** Elevator Modernization, XJ Sch****** Elevator Modernization, Xizi OTIS Elevator Modernization, Mitsubishi Elevator Modernization, Shanghai Mitsubishi Elevator Modernization, Fujitec Elevator Modernization, Hitachi Elevator Modernization, Toshiba Elevator Modernization, Hyundai Elevator Modernization, LG Elevator Modernization, Sigma Elevator Modernization, Express Elevator Modernization, GUANGRI Elevator Modernization, FUJI Elevator Modernization, BLT Elevator Modernization, CANNY Elevator Modernization, SJEC Elevator Modernization, KOYO Elevator Modernization, IFE Elevator Modernization
Elevator Modernization,Lift Modernisation,Passenger Lift Modernization,Passenger Elevator Modernization
CEP Elevator Products ( China ) Co., Ltd. , https://www.china-elevators.com