While both are classified as surface heat treatment methods, there are three main differences.
First, carburizing is performed on a large quantity in a furnace, whereas high frequency induction hardening is performed by coil on single pieces at a time.
Second is the approach toward hardening location. In carburization, low-carbon, case-hardening steel is mainly used and carbon is allowed to permeate the workpiece surface over several hours in the furnace, before it is quenched; when only hardening specific areas, anti-carburization treatment is required before furnace heating. High frequency induction hardening, on the other hand, can be conducted only on specific areas, depending on coil shape.
Third is the approach is to ensuring case depth.
Carburization case depth is adjusted by adjusting the time in the furnace—deeper case depth is achieved through a longer time in the furnace, allowing the carbon to permeate deeper. With high frequency induction hardening, on the other hand, case depth is adjusted through coil power and heating time. Adjusting hardening time by the second can lead to significant productivity gains.

Nitriding treatment types include gas nitriding, salt bath nitriding, gas soft nitriding, salt bath soft nitriding, and plasma nitriding. The heating temperature is low, around 550°C, which prevents warpage, but the treatment takes from one to several dozen hours. The case depth is generally between 0.01mm and 0.3mm.
In contrast, high frequency induction hardening heats the workpiece to above the transformation point, so warpage is more likely to occur (depending on the workpiece shape), but a case depth of 0.1mm to several millimeters or more can be obtained with only a few seconds to several tens of seconds of heating and cooling.
In addition, the hardness and hardening range can be fine-tuned by adjusting the coil shape, power input, cooling rate, and other factors.

While high frequency induction hardening and laser hardening are often confused, the two differ in heating method, cooling method, hardening area, and case depth.
As each hardening method has its own merits and demerits, the optimal method must be selected based on the purpose of hardening, as well as the size and shape of the workpiece.
The difference in case depth between the two is not as simple as "induction hardening is deep, while laser hardening is shallow.” In high-frequency induction hardening, the case depth can be controlled by changing the frequency, and depths of 1 mm or more can be easily achieved. On the other hand, laser hardening is suitable for case depths of 1 mm or less (0.3 mm to 0.8 mm) and does not require cooling. For further differences, please refer to the following link.

With general heat treatment hardening, the workpiece is heated in a furnace for a set period of time, after which it is removed for rapid cooling. This method is used for workpieces that require high hardness and deep case depth.
High frequency induction hardening is a method that utilizes coils outside the furnace to harden only specific areas, at the required depth. The major difference from general heat treatment is that the core retains the original hardness of the base material. However, to shorten the heat treatment time, a furnace coil can be used to harden the entire workpiece by high frequency induction hardening.

At Fuji Electronics, we refer to tempering that uses high frequency induction heating as “reheating.” The major difference from furnace tempering is that reheating, like hardening, can be incorporated into a line. Raising the temperature of only the hardened area to approximately 200℃～400℃ for a short time is effective in dropping the hardness level by 2-3 points. In furnace tempering, the entire workpiece is heated in the furnace at approximately 160～180℃ for 1-2 hours, imbuing the workpiece with added toughness. The choice between reheating and furnace tempering is determined by the material and the quality required for the workpiece.
See the following details for selection criteria.